If every monster inMonster Hunter Wildswere that fancy all of the time, your machine would become a volcano.
When they’re further afield, the flourishes fall away to free up memory and processing power.
Capcom ain’t talking either.

But the explanation is consistent with what I dimly understand of memory management in other video games.
More importantly, the low poly monsters are just extremely good fun.
So I won’t belabour the point this time.
Fortunately, I have other people to belabour it for me.
In amongstthe complaints about optimisation, there are people attempting half-serious taxonomies of the low-poly menagerie.