I don't think that's true. For one, the sign up of the accounts itself is often automated and in the hundreds, because bot accounts are squashed all the time. You'd spend thousands on burners.

Spam is mostly prevalent because it's cheap and easy to throw millions of requests at the wall and see what sticks. If it did cost really any money at all it wouldn't be remotely worth it.

Spammers don't just use one account to spam.

There are free services where you can see a number to get that 'activation code'. Plenty of such services out there.. (a quick search in DDG and this is the top result)

https://mytempsms.com/receive-sms-online/country.html

When twitter suspended my account after creation I tried to use these type of services. Unfortunately every number I tried resulted in an error.