Yep, 45 years old hardware, still getting software updates. Hey Apple, JPL is close to you, can you get someone to bike over and see how they do it? Thanks!
Planned obsolescence [1] in the problem. Apple could certainly create modular devices designed to be used and piecemeal modified/updated indefinitely. But it's way more profitable to sell you a brand new phone every couple of years than it is to sell you a modular device that you could update as desired, especially if the parts/connections are standardized meaning you could foster a [normalized] third party market for batteries, screens, etc.
Devices would last way longer, prices would be much lower, and it'd be unimaginably better for the environment. But Apple wouldn't make as much money, and the government's GDP growth figures wouldn't be as high. So clearly it's a terrible idea.