Because life isn't a third grade science fiction movie, where the super scientists who program AI agents are at the same time so incompetent that their experiments break out of the lab and kill everyone.
This seems to be closer to an argument from ridicule than an argument with content. No one has said anything about "super scientists"- I am however mildly curious if you are familiar with the AI Box experiment. Are you claiming that AI aren't going to get to be effectively powerful or are you claiming that you inherently trust that safeguards will be sufficient? Note that these are not the same thing.
Elon Musk submitted a comment to edge.org a day or so ago, on this article. It was later removed.
Now Elon has been making noises about AI safety lately in general, including for example mentioning Bostrom's Superintelligence on twitter. But this is the first time that I know of that he's come up with his own predictions of the timeframes involved, and I think his are rather quite soon compared to most.
We can compare this to MIRI's post in May this year, When Will AI Be Created, which illustrates that it seems reasonable to think of AI as being further away, but also that there is a lot of uncertainty on the issue.
Of course, "something seriously dangerous" might not refer to full blown superintelligent uFAI - there's plenty of space for disasters of magnitude in between the range of the 2010 flash crash and clippy turning the universe into paperclips to occur.
In any case, it's true that Musk has more "direct exposure" to those on the frontier of AGI research than your average person, and it's also true that he has an audience, so I think there is some interest to be found in his comments here.