They are built-in MongoDB types, but they don't necessarily map well to JavaScript types. Even BigInt doesn't quite line up with what a long is in MongoDB, so if you store something as an int32 in MongoDB it would still look like a Number in JavaScript.
I'm not really sure why mongoose-long, mongoose-int32, etc. has to be in their own npm packages.
These types are built-in supported MongoDB types, so people expect to be able to use them with mongoose without additional extra step.
It'd be great if this project can be bundled within mongoose.