These stealth rules actually make perfect sense if their goal was to emulate stealth in Skyrim. You go behind some rocks or into a dark corner, press the crouch button, and then as long as you stay crouched and your stealth skill check is high enough, you don’t attack, cast a spell, or make too much noise, NPCs just won’t notice you’re there at all.
The problem is stealth in Skyrim is famously absurd, and people actively make fun of the idea of others treating D&D stealth as if it worked that way.