And why rebuild Liberty Prime where the resources needed for that could be used for gear and weapons that can be used for so many more tasks and situations?
Liberty Prime is not such a useful weapon as Bethesda makes it out to be. Why not fix up armored vehicles or artillery?
There is nothing right now capable of stopping Liberty Prime of course but it can only operate effectively on certain terrains, is not really that fast, and probably requires a lot of maintenance.
It is perhaps intimidating towards the more ignorant wastelanders but a more well trained army will probably find ways of exploiting its limits and weaknesses to destroy it.
Often high tech is beaten by low tech.
This is just a little ego toy to be honest, even back during Anchorage. More a 'look what we can make' than something that is that useful.