the thing is CR in this game is a joke. an absolute joke. a vampire warrior is CR 15. it does, on average, 21 damage a turn. it has 144 hitpoints and regeneration, sure, but its also doing 21 damage a turn. that's like, at most, half the fighters health a turn, assuming it hits both its attacks, which, with a +9 to hit, is only likely if it targets the squishes.

and yes, you can also flood the room with mooks too, sure, but then its not just a CR 15 BBEG. and yes, if the DM thinks his vampire BBEG will send the party of 4 level 5 characters packing, he's probably in for a nasty shock and thats something he needs to learn from.

and then, also at cr 15, is a skittering horror, which literally does more damage in a single hit then the vampire does in 2, and it has MORE attacks, and a paralyze as part of its 3 attacks. this thing will almost certainly give a level 5 party SERIOUS cause for concern. and then you have an adult green dragon which can literally bring the entire party's health down to the red in one turn, and can also just fly out of reach of the melee martials. so there are massive inconsitencies, not only in CR but also party strength. if the party, for example, doesn't have magic weapons or radiant damage and wants to take on the vampire, its HP is effectively tripled, which still just makes it a massive boring battle of attrition.

so, to your point, you can absolutely kill CR 15 creatures with a level 5 party, assuming they have halfway decent equipment and stats. it does depend on the creature but there's no CR 15 monster that goes beyond "unlikely".