Why? Because Game Balance.

You can't increase the amount of Aptitude your MC gets. But there's no need to. Your MC has access to more Attributes than any other character in the game and access to way more traits.
Each npc only has up to 4 unique traits, but the player character can get something dumb like 30 unique traits.
If you pick all the +Attribute traits in character creation, you end up with 20-30 in each Attribute at the start.
Then the MC can get something like +15 total Attributes through several events.
Npcs can get something similar but not to the same extent as the MC. I believe Ou Ye Heng gets the most out of any npc at a total of +12 with the Calamity Dlc.

Side Note, the Old Man master, Yan Gexing gets +1 Aptitude per 2 Levels. So he gets +50 at level 100.
He's on par with a stronger MC while you have him, but the MC can get even stronger than him.

You MC will not be stronger than a max Yan Gexing on a 1st run, but once you unlocked a bunch of achievements, you'll be leaving him in the dust on your 2nd, 3rd and so on future runs.

there is an achievement that give +5 to all stats.
that just one of them. it's possible to have 45(9x5) stat point to remove the need to train your stat's. in essence you can with enough achievements start with around 15 to 24 points in each stat. int being around 24 while the other are more 15 to 18.

so that just leave 10 level up points to str and then 5 str pill for about 30 to 35 in Str.
so con 15, str 22, dex 15, int 24, luk 15... add 10 form your 100 level and 5 from your pills...

true that after around 100 to 150 achievements but... no NPC can do that.

inn lady may be a total power house if build well at level 100, but her stat will never be that level of broken. yet MC will be.
same with the sect heads.

add in a 2 yellow skill merged into a red skill and... your so broken that nightmare is like easy is for a new player.

but that will take work true.