Because it is 100 % unnecessary. If you overfill the rad, once the engine gets to the hotest it will ever get, (summer, going up a hill, with every seat filled, and the AC on if you have it), the fluid will have expanded to it's maximum, and overflowed out. When it cools down it will be at the proper level. And will never overflow again.
If you ever take your car to the track , an overflow bottle is 100% necessary. They will boot you out without it. Maybe the OP would like to run his car down the strip. I think he should consider it, as should everyone.
