A few possible answers:

1. Signal is not big enough for spammers to spend resources on
2. People who bothered to use Signal are likely to be more privacy minded and therefore less likely to fall for scam

Spammers use fake phone numbers all the time on the regular phone service

The phone calls just use fake caller ID. Caller ID is entirely unauthenticated and the recipient just blindly trusts the sender, so scammers use sketchy VoIP services that let you override the caller ID without actually proving you own that number. Work is being done to improve this: https://www.fcc.gov/call-authentication

That's means it's trivial to use a fake number for outgoing calls, but the spammers can't actually receive incoming calls or texts to those numbers.

So on the privacy front nothing has either. Good to know.

Weirdly, I got a spam message yesterday on Signal. No number - just a username.

I'm on the regular production version - not the one testing usernames.