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.
The next release of a major porn game that's expected to come out late next summer has been announced to have a ~100GB alternative to the default quality ~25GB. That's bigger than anything I've got installed through Steam, and some of those took days to download...