The way her damages are, you want to optimize damage and spell pen. Her goal is burst you, and then get out. Sort of like Zed. RoA is a good item for some AP plus some tankiness, but I would argue you want more damage. You want to get to the point that if someone is like at 75 percent HP you just delete them. Plus she can e? (I don't play her) out. Not saying that ensures survival but it increases survivability if you position and execute correctly. I'm basically saying in her kit there is an escape, which lowers the incentive to build a little tanky.
If you want evidence, just watch an LCS match. Straight damage, no resistances. In fact, some mid laners that go against LB actually don't do magic resist runes simply because with LB's damage, it really doesn't matter.