Copying into a string is a safe default. Also, proto’s API currently returns string references (and not views) so making a copy is required for open source.
(Although now std::string_view is common, I hear rumors that the proto API might change…)
Maybe that's true, but safety is an additional concern. You have way more lifetime headaches if you alias the underlying data. Copying avoids all that.
simdjson also does this sort of thing. All strings are decoded and copied to an auxiliary buffer. For strings without escape sequences in them or for end users who don't mind destroying the json document by decoding the strings in-place, these copies could be avoided. I may get around to shipping a version of simdjzon (the Zig port) that optionally avoids these copies (this sort of has to be optional because the current API lets you throw away your input buffer after parsing, and this option would mean you cannot do that), but porting this stuff back to C++ and getting it upstreamed sounds more difficult.
It does not support std::string_view (or any equivalent). Google's internal protobuf does support ABSL's string view though, but it's not public.