Optional means just that - it means you don't normally use those rules unless you've got a certain kind of group. Not thousands upon thousands who are used to the vanilla system.
When you grant advantage all the time it makes a game of D&D5 worse to play. The "optional" rules in the DMG are dodgy homebrew at best and cause more problems than they solve. Some of them killed a few campaigns I was running because everyone was suddenly overpowered, whereas before it was an interesting challenge.
It's why most of the games I've played have stuck to the rules presented in the Player's Handbook and the Monster Manual. The DMG was carried by almost no one in all of the D&D groups I attended. It was considered a poor book.