Because it's not possible. I need a ~5.5" Android phone, with SD Card, OLED, 2 SIM, headphone jack and 3 years of software updates. Not sure there is a phone like that, so I don't know what I will be doing when my current phone dies.
The Sony Xperia line up has SD card slots, headphone back, and dual SIM capability. I don't think they have OLED, but the screens get good reviews. Looks like you can expect only 2 years of updates though.
Maybe check out last gen phones that are supported by LineageOS?
That's what I'll do once my OnePlus 3 (still getting updates with LOS) dies or is too slow.
I have more or less the same requirements and the Fairphone 3+ did it for me. Only downside is that spare parts take a long time (3+ months) to be back in stock.