I want this too. Badly. There are so many amazing use cases that can be built off of directly recording mileage. And we'll continue to work on a way to deliver this well.
Believe it or not odometer is not available as one of the standard data fields from the car's data port--it is something that the individual car makers control tightly. It is theoretically possible, but we'll need to come to an agreement with each manufacturer. So it's a business not tech issue.
As for just collecting a start mileage and then summing up the trips thereafter, there's a challenge there in that we know we'll be creating an imperfect experience because we cannot guarantee that we'll have a perfect record of your trips and wouldn't want an 'estimate' of mileage that was likely to diverge from truth.