It would need to keep an open connection to all nodes from all registered people and would turn into a very bloated thing and error-prone thing.
Discussion
And easy to DoS because monitoring all those invoices means keeping state, which takes resources.
Especially when creating a zap request (i.e. binding resources on the zapper) is free, just takes sending a nostr message.
So... zaps will only ever work with big centralized custodians? That's a big let down...
Not necessarily as long some good Dev can tweak simple ln@ Foss solution.