Nvidia could flip a switch and start competing with former customers. They have the talent, the models, the HW, and they know how to quickly build out DCs.
And maybe TSMC should make cellphones? If you're higher up the supply chain why go downstream into risk? It's financially irresponsible.
Well, classically, to capture more margin for yourself. In business school they call this Vertical Integration. Samsung did exactly this. AWS too.