Because we feed the data to Js. No need for additional overhead parsing string based timestamps. Unix epoch is just an int. We don't need pre 1970 timestamps.
Pre-1970 timestamps work too:
Welcome to Node.js v12.18.4.
Type ".help" for more information.
> new Date(-10000)
1969-12-31T23:59:50.000Z
JavaScript can parse binary serialised protobufs.
Yup with external libs. JSON.parse is builtin. We do the renderin in browser so why bother with protobuf.