Two of the main resources people have are time and money. The world offers many opportunities to trade one for the other, at widely varying rates.
Where do you see people trading money for time at unfavorable rates - spending too much money to save too little time? What things should people just DIY?
See also the flip-side of this post, "what are you surprised people don't just buy?"
I can think of a lot of things that most people would be better off doing themselves, but it doesn't surprise me, since we're biased to think anything we haven't done before is really hard.
In terms of unsurprising things that people should DIY:
Just because something is an easy thing doesn't mean you will know it's an easy thing. When you figure out whether it's worth it you need to consider the chance that it may look easy but is not really easy. After all, if you don't know anything about insulation blowing, how would you know if there's some way it could go badly wrong that you haven't heard about? Also, you're ignoring the cost of getting the knowledge to do the easy thing. (Really, I'm supposed to know I have to go to a tool rental store?