How is EMI avoided inside of an IC?
Everything is really tiny compared to tracks on a PCB so loop areas and potential antenna lengths are probably 10,000 and 100 times smaller respectively for a broad-brush and sweeping estimation.

Consider an unintentional PCB loop antenna that could spuriously transmit (or receive) interference - how much smaller will this be on a chip - dimensions of area might be 10,000 times smaller hence a distance of 100:1 doesn't seem unreasonable and it's probably a lot less - consider how far tracks are from the substrate (relative ground) of a chip - 100 millionths of an inch or maybe a bit more? PCB tracks above an earth plane are a much, much wider gaps.

I'm not going to give you actual numbers because it will vary between one device and the next but just consider that to make EMI or receive it you need something like an antenna - think how small and ineffective that will be on your average chip.

Having said all of that, chips aren't exempt from EMC but it takes a lot more energy to get a chip to roll-over -  this energy will be likely more than enough to create a foul-smelling signal on a PCB track that might connect to the chip. How could you ever get a PCB track that could be as resilient as "the chip" - it's always going to be the PCB that causes problems not the chip.