Several trains of thoughts here.
In TT, 20 is the normal maximum and most features respect that cap until you get to very high level. Then you find items or earn epic boons that break the 20 ceiling and go all the way up to 30. This is a given.
Now Larian has capped BG3 at 12, which is lower Tier 3, but we're getting Tier 4 items anyway. That's CRPGs for you, although Larian could've dialled it down a bit, where's the plain +3 items?
Moving to your question, why aren't all stats getting more or less equal treatment? Good question! Ticking every box without narrative reason is a bad idea imo. They could've put a Periapt of Wisdom in, maybe they did! But that story got cut?
I notice complete lack of Ioun stones. So no +2's from those whizzing around your PC's head. Possibly an animation thing, possibly by design. 5E generally has items that grant set scores rather than adding +'s, Ioun stones excepted. BG3 has a few +2 items; weapons and monk clothes, and all for Str/Dex, the usual martial attack stats.
There's also BG1&2 legacy, Girdle of giant strength and Gloves of dexterity. Maybe certain items simply got priority. Poor Nymph cloak!
My question is, why no +2 or +3 shields??
