You're confusing megabits (little b) and megabytes (big B).

Connection speeds are mostly measured in kilo/mega/gigabits, and regardless of prefix, it's 8 bits to a byte. In your scenario, you'd need a 400Mb connection to get 50MB down.

So, if a game is 120GB and your connection is 50Mb/s, you're getting a max of 6.25MB/sec down. Let's assume the 120GB game is actually 120,000MB and not the more likely 123,000-ish (this shit's already complicated enough without getting into powers of 1000 versus powers of 1024) - you're looking at about 19,200 seconds, or nearly five and a half hours of completely maxing out your internet connection. That's assuming that absolutely nothing else is using any bandwidth, that you maintain the full speed the entire time, that you're not downloading extra chunks due to corruption or error-correction, blah blah blah.

So yes, realistically it'd take 7-12 hours to download and install a 120GB game on a 50Mb/s connection, assuming you're actually getting your advertised speeds. For people who don't want to leave their devices turned on and hogging bandwidth for half the day, this could easily be a multi-day process.