The fact that I've started using signal might not be an information I wish to share with other people who are also using signal and have my contact.
If you can't tell if a contact has Signal, it would have to default to SMS - and when sending a Signal message (to either a phone number, or in the future, a non-phone identifier), there'd be no way to tell if you're sending it to someone with Signal, or sending it into the void.
Maybe that's a trade-off you'd be willing to make, I don't think it's cut-and-dry though.