Some people aren't mechanically capable, and the cost for someone else to do it would far exceed the price of a new bike. Even the bikes I've seen that have been "tuned up" on CL more often than not still need work, especially in that price range.

Some people associate new with better, and even here there are plenty of people who will tell others that bikes older than a couple years old on CL are just junk that aren't worth it.

Some people are not comfortable scouring CL for a good find or meeting up with sellers.

Some people don't know what to look for even if they were comfortable looking on CL.

Some people need something NOW and don't have the time to wait for the right bike at the right price to pop up.