Requiring an email address is done so that a verification email can be sent. Clicking the link in the verification email proves that a live human is requesting the account. There are probably some ways around this, but it cuts down drastically on the resources needed to filter out bots and spammers. Otherwise the site fills up with garbage and it becomes useless.

I use e-mail verification on my sites as I feel that providing a phone number is too intrusive for many.