Because insanity is doing the same thing over and over and expecting a different outcome. Via The Hill:

Senate Democrats on Wednesday boycotted a committee vote on President Trump’s pick to lead the Environmental Protection Agency (EPA), preventing the GOP from moving his confirmation forward.

None of the Democrats on the Environment and Public Works panel, led by Sen. Tom Carper (D-Del.), went into the meeting room, depriving the committee of the two minority party members it requires for a quorum to vote on Scott Pruitt.

The ostensible reason the Democrats are using is that Pruitt hasn’t fully answered their questions (does this sound familiar? if not, it should). The real reason is that they know that they can’t stop Pruitt and they are simply trying to delay the inevitable as long as possible. The only question now is how long it takes for Committee Chairman John Barrasso to check his huevos and to his minority what Orrin Hatch did to his.