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?
Just because something is an easy thing doesn't mean you will know it's an easy thing.
That's pretty much why I made this thread: so that I (and others) could learn that something that we didn't realize are easy actually are.
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?"