Well while bowling googly the direction of the front leg has to be in straight direction so that the position of arm can be easily raised to at an angle of more than 60 degree with respect to horizontal shoulder level, which is absolutely key in bowling googly.

At the end of the day it is the matter of creating the flexibility to raise the position of arm a bit higher to bowl a googly.

Well some of the leg spin bowlers can still raise the position of arm higher comfortably even if the direction of front leg faces towards the leg slip or fine leg region ( which infact, is the actual position in legbreak delivery )