Plus most of the time there are so many bugs, patches, blah blah blah that need to come out first, why pay $60, $70+ when I can get the fixed/complete game later for 50% or more off?
Lots of games have bugs that literally never get fixed. Snowrunner has had an autosave stutter bug since day one which is still there. There is actually an open source patcher to fix the problem, and has been ages, and they STILL haven't fixed it, even though they could just copy the fix.
What I find compelling is waiting for all the DLCs to get released, because not long after that's done there is usually an all-in version which costs little more than the original base price. Although I broke this rule for myself and bought Snowrunner anyway, and the year 1 and 2 passes. In my defense, they were all on sale.