Plain old racing games are usually undoubtedly boring as it is repetitive and long. Adventure games never did this at any point in gaming history. The only racing games that I love are the games that don't just involve racing or at least add a twist. For example: Forza Horizon 1 & 2, includes showcase events that allow you to race against other vehicles like a train, a jet or even a helicopter. Despite this, adventure games are better than racing games because they were never so boring.
Adventure games have a lot more to them. There are things like Diddy Kong Racing, who fuse both adventure and racing in a perfect blend, which makes an amazing game. Racing games are typically best if played with friends offline. Adventure games are best played solo or Co-op either on or offline, and in this day and age, games are becoming more and more online eccentric
It' s entirely up to you whether or not racing games are better than adventure games. I personally prefer adventure games to racing games but again your own preference. Also it can depend on your mood, have friends over and want to play a game with them? Racing would be an option for that. Adventure games would be for when you want to play by yourself. My reason for liking adventure games better is when you play them again you can find new things and go on a different path but with racing games things don't really change.
And those are my reasons for my opinion.