The goal of many senators is to force Twitter to be transparent in being partisan. If Twitter tomorrow says "We're pro-XYZ and we're going to take down content

The goal of many senators is to force Twitter to be transparent in being partisan. If Twitter tomorrow says "We're pro-XYZ and we're going to take down content we don't like" then the hearings would be over. Twitter (and Facebook, et al) don't want that because they would be at risk of losing large parts of their audience. They don't really make money from a bunch of people agreeing with each other and sending nice daily affirmations. They make money when your angry Uncle Billy posts something political and gets a bunch of other people in a big shouting match - views are money. The senators aren't doing this out of the goodness of their own heart. They believe they will gain better platforms of their own uses (and potentially profit) instead of having to deal with what they perceive as an adversarial platform.