That's because they are bulk purchasing numbers from voip providers, cycling through probably hundreds per day.

Do they actually need to purchase numbers to do that, though?

I always imagined that there are certain shady providers ("grey-market Twilio" sort of idea) that just let you run single outbound call/text requests through a giant pool of numbers shared with other customers of the service. Perhaps specifically a bank of residential numbers plugged into banks of regular cell phones, like a residential IP proxy service provider.