That’s an issue inherent to the d20 system, although 5e is way worse about it. WoD and CoD have way better systems for this, even though they aren’t about combat.

Or could just use monsters that target players’ weakest saves.

Let’s say you have 20AC vs this monster +13. The monster hit on 7+. So you have 30% to avoid a hit. This means on average you’ll avoid 1 or 2 out of the 5 attacks of the big baddy. Which can definitely save your life!

So this dm never heard of saving throw attacks?

Nah, it’s more fun to let them enjoy their high AC against martial threats but bring in more casters, demons made of poison gas, etc if things are getting too easy.