There are a number of reasons, here are some:
1. You can never achieve 100% fill as roundish objects will always leave space between them.
2. Cables should not be "combed" next to each other, the natural twisting and shape of cable will take additional space of the conduit fill
3. The drag effect of pulling cable through an overfilled conduit could exceed the maximum pulling tension recommend for the cable (25 lbs for a standard 4 pair).
4. The effective cross section (area) of a conduit is reduced at bend points.
5. Heat generated by the cables in an enclosed space (not so much for data type cables)
Hope that helps you.
Joseph Golan, RCDD