Because most people who are doing it are not english native speakers? Better ask why the Anglo natives are seemingly not working on games of their own. Probably because a lot are too busy bitching about bad english while not knowing a second language themselves? Sorry if that sounds a bit bitter, but as a Non-Native speaker myself, who expended a lot of effort to learn a number of different languages, I am often getting annoyed by certain part of the English speaking world and their sense of entitlement.
While I agree that it would be way more convenient to have clear, concise and gramatically correct english in these games, please remember that unless the Dev is quite good in english themself, they will need someone to translate for them. Which is a huge chjunk of work and will possibly require expending money they don't have (yet). Later, if the game proves to be popular, they might find volunteers, but that is problematic as well as they will have to depend on people making a big effort, agreeing to schedules and so on, pro bono. Machine translation often is the only viable route.
So cut the Devs some slack unless it is clear they don't care and just try to cash in without effort.