I could understand if the complaint was about a sports game. Most of them are released annually and are genuinely very similar to their previous versions.
However, every new Mario Kart game is genuinely distinct from its predecessor. You can show me any screenshot of any Mario Kart game, and I will immediately be able to identify what version it is.
In some ways this is the optimal way for a video game company to innovate as they need ROI (people don't generally buy new IPs in high numbers even if they're really good and it often takes a couple of installments to build trust and sales!) so creating new gameplay out of trusted IPs is a good way of achieving that.