[crote]

Because the sensors aren't simple and there would be too many variables to account for.

The riders are not exact clones of each others. The bikes aren't going to be exactly identical either. You won't be able to have them ride in an exact straight line with zero wind and zero temperature fluctuation. You won't be able to track the position of 100+ riders to within sub-centimeter precision while moving at 60km/h.

The actual power measurement is easy: they already have sensors for that. But good luck trying to get them to give you that data: it's like asking a Formula 1 team for their engine performance data!