Some more objective analysis of why the Advantage rules (especially with regards to height but applicable here as well) -> https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=707485

If you think Advantage is supposed to come from your DM, I feel sorry for you. There are lots of ways to try and get Advantage in 5e (Barbarian Reckless Attack, Faerie Fire, Tasha's Hideous Laughter, Monk Stunning Strike, Rogue Cunning Action Hide, Entangle Spell, literally dozens), but the difference is those sources of Advantage come with a *cost* or a *drawback*. Currently, Height and Backstab have zero cost and no downsides, it's just all benefit.

For a sense of how game breaking that is, and how much it throws the rest of the game out of balance, here is a great example:

The most powerful resource in all of 5e, and generally in all of D&D across 40+ years of balancing and development, is a *9th Level Spell*. That is the highest magic a player can cast, reserved for the strongest spells. You can literally wipe out an entire city with a single spell. You can alter the fabric of reality. You can *stop time* with it. It is the pinnacle of player power. What is one of the most popular uses of a 9th Level Spell?

Foresight. A spell that does nothing more than give you Advantage on everything for a day, and Enemies Disadvantage against you. That's it. That's all it does. That is how strong Advantage and Disadvantage are in 5e with its Bounding Accuracy system.