Is this an assumption or coming from your experience? Because I'm typing this on a GNU/Linux phone in a desktop browser and use a bunch of desktop applications daily and haven't noticed.
Of course if you run GIMP or something like that it won't fit unless you plug an external screen and a mouse in, but all the applications I use daily are perfectly usable. There's a lot of Kirigami and libadwaita programs these days that just work well on a phone, and if I need to launch my bank's application there's always Waydroid.
I have a pinephone and try it out year after year.. Well, let's just say that there is so many areas of improvement to make "GNU/linux" run on a mobile device (that sorta includes laptops as well, even though I have done so for years) that we might as well start over from statch.
For example one can't just let everything run whenever it wants, wasting battery life. Android's "more complicated" system and binder was criticized in this thread, but that's exactly what ties together the whole thing to be able to run on a device that fits in your hand, with centrally managed "let's pause this app now" etc
There are more, not every application that works fine has metadata filled up (and not everything is on Flathub either).
I do use some webapps, but with Epiphany rather than Firefox.
See also: https://linuxphoneapps.org/
If you say, rely on google maps, banking apps, apps for your IoT appliances, etc. it's certainly relevant. I don't have any of that though.
For me the most and truest pressing issue is that cell modems are very, very tightly coupled with Android. It's still true for the Jolla Phone that it simply is a worse phone because the modem drivers are buggy. This is a complicated issue that isn't getting better, and is mostly to do with legislation legally mandating the tivoization of cell modems, a weird line in the sand on what responsibilities fall to the hardware or to what software, as well as the modem manufacturers themselves not really caring.
My impression (also for Ubuntu Touch, etc.) is that all these systems use the upstream vendors' Linux kernels trees and firmware blobs for Android.
Unfortunately, since we are not talking about Samsung or Google, but just some random Chinese ODMs, it's usually years old Linux versions and ancient firmware blobs full of known holes (e.g. the C2 is running a Linux tree from October 2022). It's only thanks to the tireless work of postmarketOS etc. that some devices boot on modern kernels.