Largely because they're residential botnets in places like Brazil (a real example from one of my sites that was crawled to near-destruction). Someone could probably do something about this, but it's out of reach for individual site owners.
How good are you personally at iteratively and sequentially jumping through the legal systems of dozens of countries over the course of years, interleaved with genuinely difficult technical investigation, to unmask successive onion-layers of identity in order to unmask one offender?
Oh, and it also only takes a few minutes to reconfigure everything and invalidate those years of legal and investigatory work.
