There's a practical reason for that; if they don't collect and store any other user information, they have no way to determine if someone trying to sign up is a bot or someone else trying to abuse the service.

I get far less spam messages on Signal than on telegram and discord, for example. There's a cost associated with setting up additional Signal accounts at the very least.

Strongly disagree. As a creator of tirreno platform [1] I can say that behavioural patterns are more helpful in fraud prevention (including spam) than phone verification, a specially taking into account that phone numbers could be bought in bulk online.

[1] https://github.com/tirrenotechnologies/tirreno