That third party app stores are subject to review by apple is a problem in itself.
That the only way to get applications on your phone is going through Apple, or a gatekeeper approved by apple, is a problem in itself too. That the first "app store" approved is built on predatory free to play gaming money is also a problem.
That's how 70% of apple's revenue is gained. AKA, the biggest players who are capable of advertising their own store without visibility from the default mode of purchase.
I'm sure eventually we'll get FDroid equivalents on IOS, but Apple is still playing coy as of now. So nothing but the deepest pockets will challenge them.