The high ground has been proven throughout history to be more advantageous in battle, is my point. My other is that it does improve accuracy. Seriously. Get a bow, fire at a target on the same level as you. Then get to a higher level and fire at the same target. I'm telling you, higher makes it easier especially when you add more targets and obstacles on the field.
Take the Battle of Helms Deep. You have thousands of orcs swarming the walls. Is it easier to hit an orc climbing a wall or at the base of the walls, or is it easier when they are on the wall with tons of allies and other orcs as well? If they are all on the same level, it is much harder to tell friend from foe and to even time your shot more accurately because everyone is on the same level milling about fighting. If you are higher, it is even easier to shoot over the heads of allies. Besides, you have the added disadvantage of having the enemy possibly coming at you to kill you while you're taking aim when in the same level. The stress alone is more if the enemy is on the same level with you. If you are higher up, you can see them better and know they can't really get you while you aim.
Anyway, I'm not saying they should keep high ground as an advantage. I just think that it is one of those things that they could make a reasonable argument for. High ground, again, is historically advantageous, so it makes sense to get advantage. I'd rather they focus on things that make no sense at all, like barrel throwing and eating food in combat and long rests being able to be spammed and actually encouraged for dialogue purposes.
Almost every sentence here was an argument for high ground's efficacy at removing cover, not the inherent benefits of high ground itself.
"...especially when you
add more targets and obstacles on the field"
"when [orcs] are on the wall with tons of
allies and other orcs"
"easier to shoot
over the heads of allies"
It's exactly as you say. High ground is advantageous mainly because it allows you to ignore cover and more easily shoot in to masses of the enemy instead of enemy+ally groups. Also, it takes longer for enemies to reach you if they have to climb a hill vs run down a hill, so you have more time to shoot them.
And 5e already has a system for cover. Shooting past a character grants the target half cover (+2 AC, +2 Dex STs). Hiding behind a big obstacle or merlons (those protrusion on top of castle walls) grants three-quarters cover (+5 AC, +5 Dex STs). Notably, neither of these scenarios grant Advantage, which means they stack with other sources of Advantage. On a un-crenelated hilltop where there are no merlons to hide behind, the bonus for height advantage should then be +2.
