Because it doesn’t make sense with their business model.
My company had some ancient capital equipment that required windows 2000. We had a contract that supported them up until a few years ago. That contract costed a fortune, but made perfect sense for the use case. Nobody is going to pay to keep an iPhone 4 updated.
Note my comment is that you’re an outlier. That doesn’t mean that your needs are flawed, wrong or anything else. It’s just not consistent with the market’s need and represents an addressable market too small for Apple.
Sometimes companies do serve niche markets by rolling them together. Sonim, for example, made a fully waterproof Android phone that was rugged and marketed to public safety and construction customers. It also solved a problem for small customers like mine who had environments where traditional smartphones could trigger an explosion.
I don’t want a phone with design characteristics the lets it operate in a grain elevator. Likewise, I have no desire to operate a phone for many years, and the market, rightly or wrongly, agrees with me.