First: They kinda sorta did in Tekken 7? That guy is dead. Not just videogame dead. Like, end of The Wrestler dead.
Second, when a VA is iconic and attached at the hip to a character, it's hard in the immediate aftermath to recast the character. I have no idea how intimate Heihachi is with his VA because I've never cared that much for Tekken, but sometimes out of respect for the person behind the character, companies will find ways to avoid using the character until more considerable time has passed.
Wouldn't be that hard for Katsuhiro Harada to say, "Hey, put in _______ instead of Heihachi," if this were a concern.