"Pourquoi boit-il autant de bière ?"
Shouldn't it be "Pourquoi boit-il TANT de bière ?" ? Because there is no comparison with anything else.
I'm no expert, but it could be an implicit comparison with what's healthy.
"as much" just doesn't make sense in English. It should be "as much as" and used in comparison. In the sense here we need "so much". (Unless this is English as a foreigner might speak it).
This sentence in English is awkward without the rest of the comparison. I would say something like " Why does he drink as much beer as his friend?" or "Why does he drink as much beer as he can tolerate?" Seems like the equivalent in English would be closer to "Why does he drink that much beer?"
tant is correct - i have found a similar sentence where autant was used instead of tant, the explication is that this is a mistake commonly made by the french.