Flanking is piling on the cheese in further two ways. How the enemy AI consistently fails to abuse it (while they exploit height advantage so much, they almost break immersion by abandoning near melee encounters to scramble to faraway vantage points), so again it's mostly a natural movement that is "player only". Secondly, the flanking mechanic is virtually guaranteed for no/little effort (unlike height advantage). The way you move to the back of an enemy without them reacting is so extremely unrealistic it again hurts immersion. It simply strips the turn-based system naked as a bad approximation of real combat. Larian could easily have restricted this by ie. having everyone pivot towards one threatening enemy within range. Realism and balance restored. Cheese gone. But actual tactics and smart plays is an impediment to Larian's silly fun cheese.
D&D is a fragile eco-system, and Larian cheese is an invasive species.
Remember, Flanking isn't actually in BG3. Flanking would be more sensible, at it requires allies to be on either side of an enemy, and it's reasonable that a surrounded enemy couldn't properly defend themselves against both attackers. It has
some limitations and risk involved.
BG3 has Backstab, which yes has all the problems that you mention.
