The potential of being a competitor in an as of yet unproven market wouldn't be a good reason to acquire them. If that market becomes very very big AND Nvidia wins it, Intel could buy them in cash if they wanted, they certainly have it on hand.

Why they haven't acquired them in the past, Intel clearly doesn't think they are worth it and preferred to compete against them directly on graphics.

Why would they? In the bigger scheme of things computer vision is still pretty much a clean slate. Nobody's making any serious money (yet). And Nvidia is still first and foremost a graphics company. Buying Nvidia for CV tech would be like buying Airbnb for its "trips" business.

Apart from that the FTC / US Justice would never allow that merger in the first place.