Easier said than done. Stark needs musters to create siege engines, and overall, more musters benefit GJ more than Stark, as Stark has an easier time with both keeping its units alive when it loses and killing enemies when it wins. Without multiple musters, GJ can find itself winning more battles than it loses and still falling behind on overall units. Related to that, Stark doesn't really want to spend muster points on sieges, since they are the one unit that does not benefit from the defensive firewall of Rodrick/Blackfish.
There's also more of a tradeoff when Greyjoy gets to act first, because it means they get to raid Winterfell or Castle Black every turn, rather than Stark getting to raid Stoney and/or Bay of Ice each time.
Overall, letting a siege engine park in Stoney means that the battle for Winterfell is a stalemate at best. And you want your capital to be safer than that.