Hi, welcome to the forum.
I'll give you one or two: adding such an EQ 'dirties' the signal path if you like. Other reasons are that EQ adjusting by ear is not necessarily very reliable, even though you may think you prefer the sound you hear. Also room interactions are vast, which means whatever you do, you will have a hard time correcting anything very precisely.But suit yourself if that is what you like, there's nothing wrong with it if you like it. However most 'audiophiles' wouldn't dream of adding a manual EQ to their system. That's my take on it,
Others will add to this i'm sure