Ground strap composition

I believe that they are made from braid for two primary reasons:

1. Durability - the braid is very robust to mechanical vibration and stresses.
2. Current carrying capacity. For your electrical system to operate properly, it is best to have the grounds tied together as well as reasonably possible. Ground should be the same potential everywhere on the car. A big strap helps this.

A good sized wire would perform the same function electrically.
If it is designed properly (so it won’t break) it would also be the same mechanically.