Well, they still can hack into Webway. Dolmen Gates are just sure access.

Also, while Webway might be larger, a lot of it was abandoned, has nasty things like Daemons or Dark Eldar lurking about, or even normal Eldar from less friendly places. I'd consider 'Necron' part to be well mapped and well maintained, at least, though Eldar raiding parties appear there, too.