Because a price is finite and unfakeable. A botnet can drop ten million likes with no actual inputs, but a zap requires actual input. It is the same concept that makes proof of work valuable. It can’t be faked.
Discussion
It can be faked, proof : several bots are zapping our content from time to time, with 1 sats or a bit more. I think the OnlyZaps is to force engagement !
You can’t print more sats. Sure they can sent zaps but it still causes them a real cost.
And the time spent reading, liking a note is certainly worth more than a few sats.