The UK ruling class is more used to bullying that negotiating to their their way. The UK had already extracted numerous concessions for being in the EU, such as the rebate, not in Schengen, not in the Euro etc etc.
The MPs are now learning the hard way that they cannot bully any more and May's deal is crap because we cannot bully benefits for UK any more.
The rest of the MPs are deluded to think that they can do any better, especially when they cannot agree a consensus.
If we do leave with no deal, we then have to negotiate a new deal (a point missed by many) which we will likely loose as our position is going to be far weaker than the EU's.