Use default id generator for user in mongo (mongoose) adapter

[Arctomachine]

First quoting documentation.

const User = mongoose.model(
	"User",
	new mongoose.Schema(
		{
			_id: {
				type: String,
				required: true
			}
		} as const,
		{ _id: false }
	)
);

const userId = generateIdFromEntropySize(10); // 16 characters long

// TODO: check if username is already used
await db.table("user").insert({
	id: userId,
});

What we do here is following:

1. Disable default id generator
2. Generate compatible id manually
3. Check database for this id duplicates manually
4. Save document with this id

Why it is bad? We disable default thing to do exactly same thing manually and get roughly identical result at expense of developer experience.
As far as I understand original motivation, we expect _id to return string, while default generator uses _id for object id and id for string id. But it is perfectly fine because adapter can easily remap id to _id right after fetching user from database.
Or is there something else that makes this simple improvement impossible?