It is because vehicles cause a lot of lag.
It is a hard cap.
Something about their location being updated on the server first, before the players.
Which means the more vehicles are used, the more the players
rubberband.
There are ways to increase with commands.
But first you should really think about if you really need that many.
See if you can convert some of the vehicles into bots like the horse or bike and stuff.
Things that don't have physics and can topple over.
You could place over 100 of those things, next to the 20 normal vehicles.