Many people even with a quad core i5/i7 might have a small SSD just for Windows and important stuff and any large game goes onto the HDD, the fact you'd do such a trade off and make the game size swell only makes the effect worse and user more likely to use HDD.
Linux (the kernel) is compressed by default (that's why the filename is vmlinuz, vm for virtual memory support, z for compression) and it doesn't impact the startup enough to have many/most distros take it out.
Simple compression might not bring lots of savings but it'll at least help a bit. And with BMP instead of PNG a game would just blow up in size to crazy proportions. There also is some (still patented?) lossy tech to decompress on the GPU, not the CPU: https://en.wikipedia.org/wiki/S3_Texture_Compression
Ooodle from RAD game tools (kind of a veteran gaming middleware company with some prominent game devs employed at it and prices being 'contact us') also has some really fast compressors and decompressors (but I've never used them and I don't want to ask for a testing SDK if I don't consider buying their product).
You can also trade time on your end for optimization that is then literally free to the end user, e.g. use pngopti.
Tokyo Dark (a VN-ish game from Japan) was notorious (for like 30 reasons but this one is very annoying) and loaded entire several hours long 500+ meg game off of the disk at once at start up for no reason other than it being simple to do or done by their Construct 2 set up. Just some compression or pngopti would help a lot for that, I remember cutting like 5 or 10 percent of game size using just pngopti.
John Carmack said that to make Rage's MegaTexture system assets work (and they still ended up being huge) he used some 200 GB server to optimize it for hours.
Crash Bandicoot also used some smart packers that took ages to run at the time on multi thousand dollar workstations (90s).
All in all: I mean to say that compression and size optimization might still be worthwhile, is cheap and doesn't have to imply a big cost to the end user, more often than not the cost is very asymmetric and the compressor pays way more than the decompressor (e.g. 7zip ultra takes hours to pack many gigs of files and uses 17 GB of RAM but the unpacking of such an archive takes just 10-30 minutes and is actually limited by my HDD speed).
