Lots of player of the game never buy the DMG so will never have access. So having rules in the DMG isn't a fix.
Also page 177 of the PHB doesn't in any way direct players to the DMG or to talk to the DM about it.
The side bar in page 38 mixed in with ability score info and gives you permission to pick a 2014 background but you have to own the 2014 book.
Self containing all the rules and options in one book makes it easier for all players and especially new players who are less likely to understand that houserules are and option.
Yes I can figure it out, yes I have decades of play experience and understand rules are just guidelines. But in the interest of making a more versatile system and and easier in future printing they should add a sidebar to page 177 to help new players.