It also has to do that just a straight up fighter without anything fancy, just taking the basic feats and spending wbl on basic gear actually falls behind monsters stat wise at appropriate CR. Of course you can optimize some which helps to a greater or lesser degree. But a core spellcaster or even like, a core rogue, simply has a lot better options for dealing with simple bog standard fights. Just like at say, Triceratops, which is your bog standard pile of meat monster. It has way more hp then a fighter of the same level, does probably quite a bit more damage, yeah it's AC is crap but it has enough of an attack bonus to probably be hitting the fighting just as easily as it's getting hit. A spellcaster, even in core, is going to have plenty of ways to go toe to toe with this thing and a few instant wins, a rogue will have some useful skills to try to tilt the fight towards himself and can throw out fistfuls of d6 at least. The bog standard beat stick fighter just hits and gets hit until one of them dies, and it's honestly probably the fighter. When all you bring to the table is being a pile of meat with a sword it really sucks when your fighting bigger piles of meat with bigger swords. It's just really sad to be an unoptimized fighter.