I don't use Signal to message strangers, only contacts who I have phone numbers of. Seems like that's the use case for most messaging apps like Whatsapp and iMessage that Signal was designed to "compete" with. Signal is a private messaging app, not an anonymous messaging app. A worthwhile distinction with very different (though overlapping) target audiences.
Why not just use a VOIP number if that's a threat model you're worried about? For everyone else, the convenience of discoverability can trump.
Is it possible to get free VOIP numbers easily online worldwide?
I have no idea. I don't purport to solve all the world's problems simultaneously, but there are people who could easily add a desired layer of obfuscation. And I don't think people should expect all good things to be free.
Why not just allow people to register without a phone number?
That's what keeps me from investigating it further.
What if you got a Go phone/burner phone for like $20. Wouldn’t that quickly solve that issue?
AFAIK you have to maintain the phone number you sign up with to be able to keep your "account". In many places, it's not feasible to keep a second phone number just for signal simply because it's too expensive.
You need to provide government issued ID in many places to buy a SIM. For example, I had to scan my passport to get a SIM card in Madrid from a vending machine a few months ago
Only if all of your contacts, and their contacts, and so on, also do the same.