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.
Not always true: for example in my current playthrough I had to choose between gaining advantage on attack with Astarion (and his sneak attack damage provides a big reason for doing so) and being exposed to subsequent attacks. I choose to attack anyway and Astarion got downed in no time. Positioning yourself behind an opponent is not always risk free, and with better AI setting up traps this could be a very real problem for those who favor high risk/high reward approaches.
Edit: same is true for climbing up high. Sometimes the AI follows you and then throws you down causing falling damage and destroying your positioning advantage (this happened to me with the Kuo-Toa encounter for example).
