Dropbox used Amazon until 2016, to only have their own datacentres in the last 5 years. Who's to say they decide that's not economical and move back to aws?
OneDrive IS Microsoft and iCloud IS Apple so they ARE BigTech, the same as Google? I don't see how that argues against the point?
Box is the only one in that seems to qualify? Kinda proves my point that it's not at all easy for a laymen.