The thickness is necessary because the insulation needs to have dead air space inside of it in order to do its job. It will not work as well if it is "squished" down or thinned. Same principle as a down jacket: its "puffiness" gives it its warmth. The recommended R-value ratings for your area are minimums: if you stray from them, you should go thicker, not thinner. That is why many insulated attics have insulation that goes several inches above the top of the floor joists so that you see nothing but insulation throughout the attic.

However, if you are finishing the attic into a liveable space, then the insulation should go between the ceiling joists in the attic, not in the attic floor. Attic floors are insulated only if the attics are not liveable spaces. Otherwise, the attic room will turn into an oven in the summer and an icebox in the winter. If you just want to finish it into storage by slapping a few sheets of plywood on the joists, I would build up a slab the size you need (a few sheets of plywood wide and long) that gives you the recomended thickness.