I’m gonna put on my political scientist hat, and point out that almost every political party on this planet enforces internal discipline in a multitilude of ways, a handful of which have been mentioned in this discussion thread already.
The idea that parties are these big tents where you can’t possibly enforce any kind of internal discipline is both a uniquely America-brained take, and also not entirely true.
Like, there are literally people called “Party Whips” who’s job it is to pressure the party members vote along party lines.