Why would manufacturers build something that hardly anybody would buy?
1) Unless it has an Apple logo on it and has the exact specs on it that Apple says are important, few Mac users would buy it.
2) 5k offers little advantage over 4k to users of operating systems that can handle fractional scaling properly, so non-Mac users would see no real value in it.
Because I'm selfish of course!
I don't like 4K because it doesn't provide enough on-screen real estate at 2X scaling. I prefer 5K for the additional pixels. However, I find the 27" size too small for 5K, unless I sit really close, but I don't like sitting that close. (I have a 5K iMac.)
For me, the sweet spot would be about 30" 5K. I know I won't get one any time soon, but it can't hurt to dream...
4K on the 30" size I'd want would not look very good fractionally scaled.
I think by chance (because I wasn't aware they were doing that until recently) the laptops I've owned since they went Retina were all at 2x scaling in their default "native" mode. And in addition laptop screens, at least from Apple, have an even higher PPI anyway than their desktop displays, so that may be helping to minimise the problem even in the cases where they are using non-2x scaling.
MacBook Air 2019, 2560x1600 227ppi
MacBook Pro 13" 2015 2560x1600 227ppi
MacBook 12" 2015 2304x1440 226ppi
I'm not certain about the default mode of the 12" MacBook, but I'm pretty sure the others were a default "Looks like 1280x800" screen mode.
That's why I said last 5 years. A lot of the defaults were changed in 2017.
IIRC, MacBook Air 2019 defaults to 1440x900 - Not 2X scaling. <-- I don't have one on hand anymore to confirm but you can check this yourself. (We won one of these in a raffle.

However, we sold it because we didn't have a use for it.)
MacBook Pro 13" 2015 defaults to 1280x800 - 2X scaling. My daughter's running it at 1440x900 though and it looks great.
MacBook 2015 defaults to 1152x720 - 2X scaling.
MacBook 2017 defaults to 1280x800 - Not 2X scaling.
Then Apple changed things again in 2021 with the MacBook Pros, by increasing the pixel densities of the screens to 254 ppi.