Why? The cover doesn't effect the functionality of the seat. The shell and the belts are the safety. The cover is just window dressing.
I do not recommend homemade covers for a car seat. First you void the car seat manufacturer's liability if there were ever an accident. All after-market products (especially homemade covers like those you see on e-bay) are prohibited by the car seat manufacturers so you are on your own if there was ever an accident and your child was injured.
Homemade car seat covers are not tested with that seat. The cover can interfere with the harness, can add extra padding that can compress in an accident causing the harness to loosen (possibly ejecting the child), and it most likely isn't flame retardent. The cover can also shift around causing extra fabric to be bunched up on a part of the carseat where it should not be, and might injure the child in an accident.
Do not risk your child's life by using a homemade or aftermarket cover. Only buy covers from the manufacturer that were made for that seat only by the manufacturer.
If you look at any car seat manual, you will find wording that indicates non-manfucturer supplied equipment is prohibited.
Here is what my Britax manual has written in the manual:
"The use of non-Britax Child Safety, Inc. covers, inserts, toys, accessories, or tightening devices is not approved by Britax. Their use could cause this restraint to fail Federal Safety Standards or perform worse in a crash. Their use automatically voids the Britax warrenty."
From my graco car seat manual in the warning section at the front:
"Do not modify your car seat or use any accessories or parts supplied by other manufacturers"
