Who buys a switch without the intention of buying the next Zelda or Mario Kart. Zelda I can understand, it's a reboot and plays differently so a demo helps.
Everyone knows what Mario Kart is and how it plays.
People who buy a switch for the next 3D Mario? I bought the Switch for a) portability, b) the hope that I could play some cool third-party ports of last-gen games that I love portably, c) playing Game Boy and N64 games, and d) Super Mario Odyssey.
There are about a billion Nintendo IPs that everyone loves. Zelda and Kart are but two of that stable of IPs.
(Full disclosure: I own both of those games, but had Odyssey come out at launch along with BotW, I might not yet own BotW, and I only bought Kart8DX because I got it for thirty bucks from Target.)