If I had to guess, and this really is just a guess, I'd say the phone number requirement is an attempt to discourage whatever the short-url equivalent of domain squatting is.

So you don't have two factor authentication set for your Google account(s)?

I love two factor auth, but I don't like that it uses my phone number. I really wish I could buy one of those RSA SecurID two factor auth key fobs from any hardware supplier like NewEgg and just register the serial number of the device with Google or any other company I want to use two factor auth with.

Even better would be to allow me to register backup two factor auth key fobs and I can just throw one or two backups in a safe deposit box, safe or similar safe place.

I think it's for Google accounts in general. Even getting a gmail account requires this.

it's not, it asks for one but you can omit it, even though it will keep bugging you about it from time to time.

It didn't when I signed up, and thus far, I've not been forced to add it.

Perhaps that's a new requirement. Some of us don't bother with mobile phones. I signed up for an account, at one time it asked, another it didn't. Try and empty your browser cache and try again.