"So, why not giant impacts triggering flood basalts?"

One needs only to look at the Moon and all the lunar maria to know it can and has happened. Although, most of these impacts occurred during the Late Heavy Bombardment 4.1 to 3.8 billion years ago.

So the questions are...

How big an impact is needed on Earth to cause such an event?

How recent have such impacts occurred?

Although, I can envision that a smaller impact in the right location could do the trick. Imagine an asteroid hitting Yellowstone, busting open the crust and releasing all that magma. Much easier than breaking through to the mantle. Just some WAG.

Brian