I think this is just because 4k has been settled on as the standard, and the difference between 4k and 5k is too small for most manufacturers to bother offering

I think this is just because 4k has been settled on as the standard, and the difference between 4k and 5k is too small for most manufacturers to bother offering both. You'll likely see 8k as the next common step up from that (it's already happening with TVs). Who knows, maybe we'll have this same conversation again then because Apple will release a 9k display at that point.