It's interesting to remember why this was possible/accepted. As I recall, at the time of its release, the iPhone's App Store was a breath of fresh air compared to software stores on traditional cell phones that were often tightly controlled by carriers and featured prices that were outrageous even by 2007 standards. For example, I remember a very crappy breakout clone being sold for ~$20 on my Nokia flip phone.
So in a weird way, cell phones weren't "locked down", they were "freed up" relative to what had come before. However, as cell phones became the primary computing platform for a generation of people the restrictions didn't lift to the point where cell phones were on par with traditional PCs and instead we've been stuck with this "good enough" situation of a mostly locked down platform with a few workarounds (side loading APKs, TestFlight) for people that really need them.
I blame a lack of competition. There has been no viable competitor to the Android/iOS duopoly since Microsoft gave up on the Windows phone. And certainly no one with the marketing chops to turn a technological advantage like "open platform" into a killer feature that would get the masses to buy in.