Why is it exactly, that mobile providers can’t catch this…? Just like with SMS fraud. It might cost them a few cents per subscriber to do effective anti-spam measures, but now society has to pay the cost

The MVNOs such as MobileX mentioned above do not have their own towers or the cellular backbone i.e. the core network. They'll merely use the MNO such as Verizon or T-Mobile and have a commercial arrangement in which MVNO just handles the marketing and the customer support. So the MVNOs may not have the right tools or data or incentives to catch these sim farms.

It depends on how closely they monitor the network and how much these people abused it. A steady, slow increase in calls/messages on a site wouldn’t show up in stats unless there was a lot of constant congestion, and even then most telcos these days outsource a lot of their network monitoring and capacity management to contractors that just don’t care.

Plus if they’re using legacy 2G/3G, it’s not the shiny thing that most telco network quality crews care about for customer experience…

Why would the mobile provider want to catch it?

If these guys are paying MobileX for 256 sims per bank * 64 sim banks = 16,384 sims and say $20 per plan = $327,680 per month of company income.

Maybe they did and informed the authorities accordingly.