D&D 5th ed has one class that can literally switch advantage on and off as it sees fit (barbarian via reckless attack), that's how difficult getting advantage in D&D is. Sure it's supposed to be ballanced because then enemies get advantage hitting the barbarian as well but then you just need to be a bear totem barbarian and you get resistance to all damage but psychic. At the same time there are low level spells capable of giving enemies disadvantage and they only cost a sppell slot.
The truth is a good part of 5th edition is about gaining advantage on attacks and that's not so difficult to achieve.
Every way to gain advantage has either a resource cost (a spell slot), or a drawback (Reckless attack gives enemies advantage on their attack rolls). Walking behind an enemy does not cost a resource or have a drawback. Neither does being up high.
