You can only reach a few people before an account is banned due to reports.
And most people won't fall for scams.
And you also wanna pay the remaining operation not spend most returns on signal accounts.

Because of that there's a massive difference in the kind of spam you get when it costs cents vs a few bucks per account.
I definitely get more spam on discord than signal for example.

3 bucks is also a lot more than it takes in some places to get numbers, so maybe long-term signal will try to also limit those number spaces with a fee? Either way this is enough to keep numbers as the weak link not numberless signups.

I don't think the same credit card can be used for multiple accounts.

So it is not about money but hoops one needs to jump through to pay that money.