It's not just the limits imposed by Mega (which a lot of developers choose to use as a distribution platform); it's also the time taken to download, the bandwidth consumed, etc. Not everyone lives in a major US city with cheap super-fast unlimited broadband.
Internet access in America sucks even in big cities when you compare to Europe and some Asian countries like South Korea and Japan. In fact ironically rural Europe tend to have better internet access than cities because digging is so much cheaper when you don't have to care about sidewalks or anything else like that, which needs to be restored. This means it might be easier to get fiber in a house in the middle of a field than it is inside a big city. The percentage of houses on fiber also tend to go up if the 4G/5G coverage sucks.
Having said that, I would be very much against lowering the quality for the sake of download size. If we are ever to be serious about that, a better solution would be to have the ability to upgrade. Imagine having one file for each release, so to get a new version you can download a addon file you add to your existing game, which adds the new graphics. The script file will then have to be overwritten each time, but being pure text that's a few MB. Using this approach you don't have to download unchanged graphics each time.
The problem with this approach is that it makes development more complex and will probably slow it down. Commercially this approach is usually used with finished visual novels where the addon file is bugfixes rather than added content. As such it's much less of an issue that it makes script writing and hence development more complex because it's way more complex to figure out where each image file is located.