Reality forces people to do that. When 90% of people do whatever they use programs for via mobile, you (as in, a business who wants to succeed) are forced to make an android and an iOS app.
Plenty of apps are Android only or iOS only. The only "force" is that businesses see these platforms as worth it to make enough money to be a return on investment.
Yes, accessing more than half of the market in US and about 25% of the worldwide one is surprisingly something businesses want to do.
Of course businesses do it because it's profitable. The point is that the market is ridiculously controlled while you are pretending that businesses can make _free_ choices, while for many of them they are not free or in some cases not even choices.
