Speculative questions:
Maybe there are some number switching, chip level protocol mitigations inherent to the standard?
Maybe carriers can push locks or there is remote attestation that could inhibit more than 4 registrations?
Will read up. I've never read the eSIM standards.
Though yes you are correct, if you're only thinking of a single chip (eSIM) instead of the system it usually goes in yes, you are be correct - those systems can currently exist.
iSIM, integrated eSIM with SOC (cpu, gpu, ram, wireless baseband, eSIM) will eventually be more common in phones.
Look up: "remote sim provisioning"
There are SIMs (either physical or eSIM) that do not have a phone number and/or has restrictions on what it can do. It's not the form itself that limits the functionality, it's the mobile network provider who decides what functionality each individual SIM has.
E.g. Mobile card payment devices may have a SIM that has no phone number and is data only with connection only permitted via a certain gateway to the payment provider.