What's to stop tsmc from designing its own chips?

I've never designed a chip before and don't know what's involved in designing them, but what if TSMC, in addition to being the world's foundry, decides that it wants to design chips as well. I imagine that would devastate NVDA, AMD, INTC, AVGO and so on, right?

Is there a reason TSMC doesn't go this route? Is it a contract that they sign with the chip designers? Or is it because they don't know how to design chips?