They tend to be both formulaic and ritualistic.
This makes for a standard evening.
Everyone has to wear the costumes, which is fine, but not everyone looks as good in the tee-shirt as everyone else.
The problems come with the ritual, because a large part of that means consuming copious amounts of alcohol, and then being loud and lairy in the same way that lads are on a night out.
The really big problem is when the party not only has to convince itself that is having a fabulous time, it also has to convince everyone else in the surrounding quarter of a square mile as well
This is achieved by being as loud and raucous as possible, and in the case of inebriated women trying to convince party members and strangers alike just how much of a really good time they are having, which is demonstrated by outdoing everyone else in the raucous behavior and industrial level laughing and shrieking, this becomes an issue for non-party people in the immediate and less immediate vicinity.
As I say, formulaic and ritualistic in equal measure.
No-one wants to appear not to be having a fabulous time, and risk being talked about in negative terms for the next twelve months, so everyone turns up and does what's expected.
One reason why I don't do hen party discos!!