Because it was seen as easier to use X.509 (aka a certificate) as a delivery of a Trust Anchor (specifically, a "subject name" and "public key" pair) than to invent yet another storage format.

Certificate verification stops when you encounter a TA. Some libraries do the wrong thing and check that the terminal cert is self-signed, but that's not actually required (nor recommended). You just check that the previous cert is signed by the TA, which involves checking the previous cert's signature (aka the intermediate) with the trust anchor's key.

That's why the trust anchor's signature is irrelevant.

See RFC 6024 for a discussion of terminology and concepts.

Simplicity and uniformity?

There's less chance to screw things up if the spec says that root certs should look exactly like all other certs, rather than trimming out parts that shouldn't be needed.