https://en.m.wikipedia.org/wiki/Rise_and_Kill_First at least one group of people appear to have accepted at least some of your argument.

Furthermore, assassinations fall into three categories:

Where the assassin takes credit afterwards (for intimidation, bragging to supporters, etc), where a third party is blamed (to prevent reprisals being directed at the source), and where it is unclear that an assassination was performed (wow IBM got screwed hard by that plane crash).

From the perspective in the OP, it is clear that there is a detection challenge. The most useful categories (to an assassin) are the third and the second, the least useful is the first. An external observer will see only the first category, and a potential subset of the second category, but is unlikely to see many members of the third category.

Maybe they're very common, and you're just not seeing the obvious.

The intersection of people with the skills to succeed at (and get away with) such a murder, the skills to predict whether it will lead to their preferred financial or political outcome, and the psycopathy to think that it's worth it, is vanishingly rare.

All of these things are easy to underestimate:

• Those who are at risk of such, are generally aware of it. They take layered precautions that you may or may not be aware of.
• It's not as easy to get away with it as you seem to think. For an impactful result, technology and data science be used to find the perpetrator. And they'll work backwards from the result, too - unusual trades in the stock, or political groups that benefit, will be very heavily scrutinized.
• Reactions will be COMPLICATED. The stock may rise rather than fall, if it concentrates attention in such a way as to support the company, or if they have the succession narrative ready to go about how it makes them stronger.
• There are near-universal social norms against it. Even if you don't get caught by the authorities, you still have to hide it from your acquaintances.
• There are near-universal individual installments of those social norms. Most people can't or won't seriously pursue this in the first place.
• Most people with the resources to profitably pull this off _ALSO_ benefit greatly from the norms that protect them. Violating those norms weakens them and makes the profiteer more of a future target.

Off the top of my head:

1. People willing to commit murder are typically those with little to lose. In order to make a significant amount of money from shorting, you have to have a significant amount of money already. Yes, rich people often want more money, but they’re unlikely to tolerate the high risk of committing a murder when there are many less risky ways of making money.

2. If the USA assassinates foreigners, foreigners can fight back. It’s in everyone’s best interest to maintain a low-assassination equilibrium instead of a high-assassination one.

3. Some of this is done in war. Why not outside of war? See 2.

Political goals seem ripe for assassination

That is a huge misconception. Can you name a single US assassination of a foreign head-of-state, in the last 50 years, that didn't blow back on us? In every case I can think of, where the US has assassinated a head-of-state, the state has either ended up collapsing into instability or has eventually replaced the leader with a leader that was even more hostile to the United States.

Also, depending on your model of history, assassinations may be completely ineffective. If historical events are the result of large historical trends converging, then assassinating any particular politician might shift things around by a few years, but may not actually stop events from occurring.

Political goals also seem ripe for sabotage.

Also incorrect. This was tried in the 1970s by a number of far-left revolutionary terror groups, both in the United States and Europe. Weatherman, Symbionese Liberation Army, Red Army Faction, the Italian Red Brigades, all tried overthrow their respective states via a campaign of terror bombing and sabotage. They all failed. The book Days of Rage, by Burrough, chronicles many of the American revolutionary groups, and how they were all eventually either hunted down or scattered by state pressure. (If you don't have time to read the book, David Hines has an excellent summary on the blog Status 451).

As it turns out, nation-states are pretty resilient, and can remain functioning even in the face of enormous pressure, and a guerilla campaign of sabotage and assassinations hardly constitutes any pressure at all, much less enormous pressure.

Even psychopaths are risk-averse. Why take on the physical risk of performing assassination or sabotage when you can take on a much lower risk (for similar reward) via white-collar crime.

I sense it is mostly because people naturally refrain from murder unless it is seen as a last resort measure, or has hugely positive consequences.

Because if you have the kind of intelligence and common sense to clearly see and care about the wrongdoings of politicians, this same intelligence would suggest the idea, that ending the life of one or several of the endless supply political miscreants, would be fruitless. The reward would not balance out the risk. Killing a person is possible but killing stupidity and/or evil is impossible because it is a scale.

The only hope seems to be to continue to permit these self-centered, ego-inflated, ignorant (or often intentionally evil) politicians to publicly broadcast their stupidity, and cruelty so all people can easily see their true colors and promptly dismiss anything they say or do as incoherent babble and self-aggrandizing actions.

While i dont have a definitive answer, i will say one of the reasons that left wing political movements nowadays (Occupy wall st, BLM, the environmental movement) dont have the same pronounced leaders that the left wing movements of the 1960's did, is due to the targeted arrest, harassment and assassination of those leaders by authorties.