Benjamin Olson
Hero
One thing I'll say about 5e monsters is that their Perception scores are universally easily outpaced by a player making a Stealth-focused PC. That may be design working as intended – i.e. playing up the trope of heroes sneaking past without raising but it's something I've noticed.
I think it is definitely a design choice, at least to an extent, as it accommodates a playstyle of often circumventing encounters in various ways which many people like. And it is more than fair in many dungeons since they are a) taking a calculated risk that whatever they sneak past won't come to gang up on them when they eventually trigger a general alarm, and b) making investing in stealth.