Most consumer & enterprise web services are international, and making an implementation for every jurisdiction is burdensome.

There are also some ugly edge cases - what happens if somebody is too young to get a SSN equivalent (e.g NI numbers in the UK), or somebody expatriates, or if some government allows people to request a change to their SSN-equivalent, or if a customer is a refugee without an SSN equivalent?

I do think SSN-equivalent identifiers may be useful for services which are inherently for tax paying adults like some accounting/banking services or marketplaces.