Ummm, because that would not be constructing?
The very real and excellent reason is that there is no game reason to do so. It is a GvG feature that makes no sense in GBG. In GvG there is a cost to the guild for a siege. In GBG there is none. The only excuse for it would be due to alliances, which is a player construct, not a game feature. You could say that GBG is closer to reality than GvG in this regard, though. In reality, alliances can be ignored by individuals at their discretion, just like in GBG. There is no real world equivalent to the GvG level of guild control over players' actions.
To sum up, guilds in GvG are used to the player construct of alliances. GvG has guild control features that give guild leaders complete control over their members' ability to attack sectors, thereby enabling the player construct of alliances. Since there is no game feature marking guilds as allied (and it has been proposed from time to time), it seems clear that Inno is not interested in giving further alliance-enforcing tools to guilds. The fact that GBG has no such controls bears this out.