I suspect it has to do with the focus on tactical combat in 3e/4e and the shift from exploration-based gameplay to story-based gameplay. Instead of a game involving clearing dungeons and getting treasure, the game became about the combat itself and dramatic scenes more suitable for a soap opera than an RPG. Look at how 4e handled "boss" monsters. They took the trope of an entire party vs. one super powerful creature and coded it into the system. Instead of boss fights that were dramatic based on the number of combatants, 4e designed a system ported directly from videogames, shapeshifting HERE IS MY TRUE FORM monsters and all.

Hahah, I've found Fighter is actually the most complex class to play effectively.
 A wizard gets a list of spells to pick, simple, a Fighter has to figure out on his own the best way to hit with his sword/bow without getting killed by the levitating drow/medusa/fire giant/dragon that flies and knows magic.
 Or I wind up raising an army and playing an RTS subgame :l

 So yeah, I really don't understand it when people say "Simple Fighter". You might as well be playing a videogame if all you do is press the attack button.