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Does anyone know why steemit has the only wallet on the internet that does not offer 2fa? Is there some sort of technical limitation that I am unaware of here?

I'm not sure if and how classic 2FA would work her. As I understand it steemit.com is just front-end for the blockkchain and does not handle classical user accounts and authentication on server-side.
You basically enter your private-keys (or the pw to create priv-key from) in your browser and then sign transactions (like upvote, post or steem transfers) with that key in your browser.

In a way it already has sort of a multifactor-auth with the role specific keys,.. just remove the owner and action key from your browser and the worst thing that could happen if you get hacked is someone posting/upvoting with your account, but no steem transfer without owner/active key. I even put my posting key on my mobile, which I normally not trust with cryptos.