It's very hard to track where all the satellites are. Military satellites generally have manoeuvre capability so keeping track of them can be quite challenging.

Also launching an anti-sat missile is a comparable problem to launching a satellite into the target orbit. A bit cheaper, but within an order of magnitude. That makes high orbit satellites, particularly in GEO very expensive and time consuming to target.

Put it this way, the US and Russia have had decades to put up all their satellites. You're not going to do enough launches to take all of that out in a few weeks, or even months.