How is this not? Steam clients are basically glorified web views for the Shop portion and user regularly enter credit card details in them to buy games or hardw

How is this not? Steam clients are basically glorified web views for the Shop portion and user regularly enter credit card details in them to buy games or hardware. User also might have virtual currency or valuable stuff in their account worth a lot of real money and users trust everything that's inside theses clients. So by MITM any unencrypted connection to Steam server either credentials or payment info.