They can't. They are still playing catch-up. ARM chips aren't stopping improving, Tegra 4 was announced and The new SnapDragons also.
And then comes markets. ARM is every where. Intel is not.
Personally, the only reason I see ARM as limited in the PC space is OS support. Put Win8/Pro and Mac OS in an ARM, now what?
But sooner or later something will happen, and I like that. Its not boring anymore.
In a few years, maybe in 2014, will see Intel vs Nvidia vs AMD vs Samsung vs Qualcomm vs Apple vs Whatever else. Everyone is doing "cpu" and "gpu" now. We will get better products with all that competition. I think.