Because female squires usually lacked the strength a male would.
Though they would normally have stronger blood arts or mystic abilities and agility, their physique would still be that of a woman, and thus, unless their bloodline or master were extremely powerful, they would be weaker.
The strength didn't mean the master's current power but their potential.
All living beings, be they monsters, humans, or beasts, had a limit. Whether it was a limit of ten or a thousand, they would eventually struggle to grow stronger.
However, this wasn't the case in the beginning—when the old gods roamed the world as incarnations. Their first spawns could grow almost limitlessly, which led to the first great fall of the supernatural races.