We actually had this happen somewhat early on in our game. The bugbear chieftain used his intimidate to frighten some of the PCs who failed in their WIS save versus the bugbear chiefs intimidate skill check. It also bolstered the other bugbears and made the combat pretty intense. Of course, we were lower level then.How do monsters even use the Intimidate skill? Is that really worth an action or better to just ATTACK and let that be intimidating Like a bugbear chief has intimidate +2. I can't remember ever using these skills
Our last session was raid into a complex of gnolls. We're 9th level now, and you might think it would be cakewalk... well, it wasn't "hard" but we had to do a lot of planning, avoid large groups, stealth/scout the best path, take prisoners to get intel, and more. Even then, it was still hard at times.
All in all, the best of PCs should beat the best of monsters, but most PCs aren't the best. One or two might be very good with perception, but many others aren't.
In an ambush encounter, don't use the best roll for the PCs. Each lives and dies by their own roll. Our first rounds typically have one or two PCs unable to act due to surprise when ambushed.
I know our DM inflates some skills when making checks vs. PCs, but not a lot. He also grants proficiency to monsters that should have them on a case-by-case basis. However, I agree that some of this seems like laziness from the devs not to give monsters 2-4 skills by default each. It isn't hard to do on the fly, but should have been included from the get-go IMO.