But most of the time I'm getting a 2-factor text, I'm at my desktop. I think an emailed code would serve much the same security benefit and potentially be more convenient.
You could even let the user chose if they wanted to receive it via email or text each time they're challenged.
Even if your not stupid enough to use the same password for both services, you might still be susceptible to key loggers, malware etc... meaning again that SMS would be a better option.
It's fairly easy to forge an SMS originating number to make a text message look like it's from someone else, however 2-factor generally is sending (terminating) a text message to a known number, this is much more secure.
It's almost impossible to intercept a terminating SMS, this is down to how SMS are routed over mobile networks and the SIM card registration process. Basically you would need the private encryption keys of the mobile operator to clone the SIM card and create a fake registration for that number, or 'root/admin' access to the current network that subscriber is on.
Same thing can happen with a phone but, usually, when someone has their phone stolen, it gets quickly reported and disabled. When your email is compromised, you may not know about it and even if you do, how long will it take to disable the account or recover it?
Using Email or SMS are equally bad and frankly its actually easier to send a fake SMS. That said, its contextual and SMS is usually faster, especially if you use gmail/etc.
Without an actual OTP or TOTP token, its basically just window dressing. And to be completely real, I'd worry more about an exploit of the site's code than any sort of 2FA issue.
Honestly, I think it's not being used because it's not sexy.