At work we are required to call customers with a non trackable phone number. So everytime we call someone, it only shows "unnknown number". I don't get it how most people would answer that. I would never do that.
Lots of reasons, none of them excusable. They could want to stop people from calling back and are too lazy/can't figure out how to set up an inbound call back route to go to a "Don't call us back at this number" voice message; they could have switched phone services (or they're utilizing a call center/got a new call center) that masks automatically. Those are the two that spring off the top of my head.
I don't, 'google screen' answers it and transcribes the call. Funnily enough I've not had a single call transcribed. So that's a search and block/report.