Because they don’t want to?

It’s not just about money. It’s complexity, company size, management, etc.. Loss of focus by having to build a new app from ground up. Features and improvements take longer as they have to be done twice. Parity problems. Support debt. Maintaining multiple versions of the same app isn’t just “hire more”.

As you agreed with, they are successful. Maybe they’re happy with that.

I'm sure they can afford that, but would that end up paying for itself? Would that end up bringing in enough new paying users to justify spending all that money on a team of Android engineers?

They probably haven't done it because it doesn't make sense for them financially.

It’s not that they can’t afford it, it’s that Android users aren’t worth the investment.