As a devil's advocate, how about the notion that requiring mobile numbers mitigates, if only a little, automated spam?

In my country you can easily buy a few thousand SIM cards for a cent or two each. I've seen them in classified ads, they're not even hiding.

Same in mine, spammers buy blocks of numbers (for example all numbers from XXXXX100 to XXXXX999) and use them to avoid being blocked by people who block individual phone numbers (with the default phone app). Fortunately I found Yet Another Call Blocker on F-Droid and I can block by pattern with it (XXXXX*) but most less technical people won't be doing that.

You don't even need that, at least in America. Phone spoofing is terrifyingly easy.

Do you not have to pay a network provider in order to use it to make calls and send messages? How does this work?

Personally I don't really mind receiving too much info (asid from DoDS attach of course). Bayesian running locally is pretty good at filtering out spam any way. Stop pretend to corporate nannying us.

The problem with that is, if you are going to automate spam, you can afford a burner. Or heck, just pay the $16 or so and hijack someone else's number.

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.