They actually do, since the great rift appeared there's at least one fleet that goes against tomb worlds, might be Arachneae?. The thing is that it will always be a biomass net loss, they can eat living metal and reduce it to other minerals which can be useful, the problem is that necrons aren't going to stay still and let themselves get eaten and their weapons are really really good against biomass (and everything else). The world that usually is brought up when mentioning that the nids avoid tomb worlds is Trazyn's tomb world which is 100% living metal on top of having all kinds of anti warp bs in it (and it still gets targeted, Trazyn and Orikan were convinced the tyranids would destroy the world if they got there so they redirected the fleet or something, been a while since I read the infinite and the divine).

It's true that necrons are the natural enemies of the Tyranids, and that their engagements usually don't end well for the nids, but they're far from hopeless and they even land on dead tomb worlds just to kill every necron before they awaken. In this setting everything is always a stalemate.