because they take no room.... for example... all Apple does for Apps, music, movies, and some others is register which ones you have a right too... so they aren't actually 'backed up'... you're just allowed to download them again if need be. IOW if you and I both own "Midway", the movie, the same copy is registered to both of us so we share the same 'stored' location.
And most files take up almost no room... spreadsheets, word docs, those are TINY.... the biggies, as you know, are home movies and photos. I failed to mention.. those are so big that I do NOT store those online... it would take a month to upload them, maybe longer... those are only on my externals and if I need to use them on a separate computer, or device, I transfer by USB Stick.