Right, but even when the panels are the same you can still tell the difference between a Sony 4K Oled display, and Samsung and Apple 1, and a Motorola one. So there are things being done to improve the visuals of the display but those things can't be done the same way of the res is lower.
Sure, they could figure out another solution but you also don't realize phone makers and carriers partner up and intentionally make it so you have a damaged or dead battery in 2-3 years to force you to upgrade. just recently in the US, several towers supporting other signals were taken down except 5G and LTE making most phones bricks overnight, and that's been gradually happening across other countries to. You'll have in one year a battery that loses 25% of it's capacity, wear and tear, processor slow down through intentional updates, and otherwise.
They don't want you to have a more stable phone, a more durable phone, or a phone with better battery. They want you to have to waste money and they had to kill a bunch of active phones recently to be able to finally deal with the problem of laggers.
Sony uses 4K displays constantly to justify the prices for example. So do other brands just having OLED whether it's 1440 1080 or 720p. Once you realize these companies aren't thinking about you realize why many don't ask the same questions you did. Because they know it's all a game to nickel and dime.