These are just examples, I have no opinion on what is the best.
Something like: I like the cameras from the Galaxy s23, the processor from the latest Pixel, the memory from the Razor. I mean whatever. I suppose Iphones could be included, but I figure it’s more locked in than androids, I could be wrong.
Or even replacing a part from one phone with one that’s better, for personal use? Like, even just putting pixel 7 cameras into a pixel 8 phone.
Besides the factory warrenty, and money spent, is it software? Is it having to reconfigure the hardware? Is it just space in general?
If we all have things we don’t like about our phones, why aren’t we able to just make it more to our liking?
You are, but you have to figure out how to get it all to work together yourself. That means physically, electrically, and in software. All the things you listed are problems that need to be overcome. If you have the ability, you can do this, but you yourself have to do it or find someone who specializes in such particular work. Most people have neither the skills, time, or patience to do this.