That’s a different problem. Regular radar is setup to detect flying aircraft and is very useful to avoid flying into mountains, avoid mid air collisions, etc and it’s not what we’re talking about. That type of Radar has proven necessary on many occasions.

Spending a great deal of money to track aircraft on the ground however is only helpful when one of the aircraft is on the ground. It’s only partially useful when visibility is low, at which point air traffic control can be especially paranoid.

It would interest me to know what the reasons are for ground radar not being more widespread and how many accidents are preventable by ground radar yearly. My first guess is that incidents where ground radar would've prevented them are so rare that it isn't worth the expense or the additional complexity or additional point of failure, and it's more productive/effective to solve the issue at different points in the system instead.

This is like saying "I don't wear my seatbelt because I've never been in an accident". Safety systems isn't about one or the other, it's about having multiple systems and procedures in place to prevent a disaster. Proper procedures are good, and it's a first line of defense, but humans make mistakes and are imperfect in other ways (Such as not being able to see through fog). Proper procedure in the case of near 0 visibility would be to greatly increase separation between planes, something which wasn't done in this case, and something which would slow down traffic a lot more than passengers and companies like.