Because a protocol with a finite number of coins has an inherent value to it, by virtue of the fact that other people are there and you can only use it if you have a token. Imagine I have a club, and it costs $5 to enter. You can always just create your own club for $2, but it's not a given that people will come. This is how a completely open-source protocol can monetize.
The same is true with Bitcoin or Ethereum. You can fork it and make your own, but the majority of the people want to use one of the original protocols, and you can only use the protocol if you pay with the proper protocol token (either BTC or ETH).
The same is true with Bitcoin or Ethereum. You can fork it and make your own, but the majority of the people want to use one of the original protocols, and you can only use the protocol if you pay with the proper protocol token (either BTC or ETH).