Stefan Amaris is just such a character per his description in the Liberation of Terra series. Regular gunnery/piloting, crazy force multiplier. Vasily Sandoval in ER:2750 is pretty similar. They exist, but they're just not used often *enough*.
There are plenty of leaders who aren't great MechWarriors. We just don't get their Piloting/Gunnery skills very often because, you know, they're not great MechWarriors and are unlikely to be used in a game. (But Sun-Tzu Liao had better be one of the pilots for the
Emperor.) Keep in mind that so many leaders are exceptional fighters is a deliberate style choice for the universe, to reinforce its nonfeudal setting. Early writeups for the fictional call the heads of the Great Houses the "Successor Warlords." A person who wins often in battle is portrayed as more able, even justified, to rule. Devlin Stone became a king by his own hand, for an extreme example.
Now that said, the truly greatest MechWarriors - folks like Yoringa Kurita, Morgan Kell, Kai Allard-Liao, and Natasha Kerensky, the genuine 0/0 pilots - are portrayed as being so deadly that they're above politics. Of these terrifyingly exceptional fighters only The Black Widow held real political station, and that was part of a game being played by Ulric Kerensky. This also fits in with the aesthetic; a Baron or
shogun could swing a sword with skill, but their greatest knights and
samurai were exemplified by being deadlier than even their lords.
