Because your email address is load-bearing here. It's how we verify the account is yours, where security notices and account recovery go, and — this being a correspondence assistant — part of the product itself.
Disposable and relay addresses break all of that: verification links evaporate, lockout warnings vanish, and recovery becomes impossible. So signup requires a real, permanent address. We'll treat it with the respect the Privacy Policy promises — it's never sold and never used for advertising.