If you get the game on Steam or Itch (using the Itch download app), they do that actually. It just downloads new/changed files, which makes for much smaller updates. I don't think many people know about the Itch one.
Most devs can't do that on their own, though, without nontrivial coding knowledge/experience that many of us don't have. Most dev tools don't have a built-in option for that either, so it'd be 100% on the dev to separate out files and/or set up an automated tool to do it for them, and frankly, I don't know jack shit about coding aside from what I've learned working on my game.
It's also a lot of work for the dev compared to "have Renpy make my latest build for me" when release days are already busy and stressful as fuck. I've worked 16+ hours on a release day before trying to catch and fix bugs, I have NOTHING on my brain by the end of it. It's a pleasant surprise if I can even hit "upload to itch" on the renpy launcher by that point.
The more minipatches/versions floating around, the more likely that someone will apply them in the wrong order, skip some, or not understand the system at all, and all of those immediately become the dev's fault and the dev's problem. Constructive criticism and real bug reports are super helpful, but PEBKAC errors are extremely time-consuming to diagnose, especially since people tend not to admit they save edited or maybe missed a patch somewhere that is causing all the issues.
So yeah, as a player I definitely get where you're coming from and love Steam especially for doing that, but as a dev it's a *lot* easier to just have self-contained versions that work right out of the box, and let Steam/Itch take care of the whole "download the new bits" part.
From a dev perspective, I think it's better to look into ways to minimize file size - using webp instead of png, careful quality compression to minimize file size, etc. Taking a music file from 70mb to 2mb, or a clip studio file from 200mb to 1mb in webp files, helps me keep DD's size down pretty small. So a steam/itch patch can be measured in the kb or "a few MB" range.
Some others have posted great comments while I was typing, like the internet connection thing and compression, so I'll just end this here.