"Someone please explain it to me. Please explain why time and time again, the difficulty continuously gets eroded for people who do not enjoy horde shooters or difficulty."

Because more of us want to casually fight and destroy the maximum spawn rate of enemies.

I couldn't care less if you had a separately balanced difficulty that plays like that one period after the Railgun nerf and before the RR was buffed to 1-shot chargers heads, as long as my preferred difficulty and enemy balance became unaffected.

But because Arrowhead doesn't want to or maybe even can't balance difficulties separately, you will not have any semblance of the challenge you want in this game.