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!

They all already have power meters! It seems like you should know that 2 riders of similar weight and profile are doing vastly different amounts of work, and extrapolate from there.

Of course, you won't be doing 5-10% the output because there's still mechanical losses and rolling losses, but they should have decent models of those as well.

They can use the computer sim for many other things.

Not to mention they don't have to hire or coordinate any cyclists.