The very question as to whether Mario Kart was needed on mobile was addressed a while back and, hey, it's an interesting opinion when looking at Angry Birds GO. If you want a good laugh and great content just take a look at those comments.
In all fairness the most valid reasoning for Mario Kart 8 over Angry Birds GO is that it doesn't have Luigi's death stare - something that MUST be in Mario Kart Tour or I want my money back already.
Of course, the classically satirical British humour doesn't rub off well on everyone, so take it as you will. We had fun with it and we hope Mario Kart Tour will also be a good ol' time when it launches.