As we all know spam text messages aren't a thing since it's very costly and hard to spoof cell phone numbers.

Those kind of SMS check usually ban voip providers. So you need a real line or a prepaid cellphone, and it can presumably only be used once or twice on this service before the number is "burned".

Such checks, although not perfect, do prevent a huge amount of spam and bots by having a very real cost for bots/spammers, and essentially no cost (other than their privacy) for the user.

Even a one cent toll is enough to change the economics of many bot behaviours.