To be fair, protobuf is a pretty poor/annoying match to many languages that aren't Go or Java, including TS. Optional types, for example, are very much not unique to Rust.
But it's good enough and has very wide and mature support for many languages and a good story for backward compatibility which is important when you have files you want to read 2 years from now or when you have different version of your clients and servers co-existing.
There _could_ be a better interchange format in theory (and many have been proposed), but a combination of maturity and network effect (and the fact that many of the alternatives to protobuf do improve some things but do other things worse than protobuf) make it very hard to chose an alternative.
EDIT: ah, about discriminating zero value from unset value (aka options), they're back in proto3 (experimental) https://github.com/protocolbuffers/protobuf/blob/main/docs/i...