Another reason is change of generations.
Many old style AAA developers see using off the shelf middleware as heresy.
Many old style AAA developers see using off the shelf middleware as heresy.
Also triple AAA isn't that old! There's not a generation retiring quite yet :) People who want to still be in the industry are. I've seen the industry get older on average as I've worked in it.
Forms of middleware have been there since the early days of AAA style games - sound, video encoding etc. such as RAD Game Tools
If I recall correctly, Tribes was one of the first ones to have some moderate adoption.