I can accept bad wannabes over this Japanase VA monopoly. Atleast then people can hope for better. There is an abundance of Thots with good mics in the west. I'm pretty sure some of them would be willing to take on these projects. We have RPGM mod and translation patreons, why not voice mod ones?
Because VA is something that requires some talent and casting (capacity for acting and a voice that matches the characters) and is generally expected to be paid. Translations and modding on the other hand are something that anyone with the knowledge and the spare time can do as a hobby for the fun of it.
Also to a big part of the audience (me included) the original JAP dub is generally better than any other VA even if they were quality ones (which since this not a mainstream genre its quite hard to find, not many with actual skills wish to give their voice to hentai).
Japanese VA as a monopoly for their productions is a given and as a proud people they have always given a premium to experiencing content in their own language nurturing this way a BIG VA pool of talents that covers all kinds of roles in the entertainment industry.