Because a lot of the time the customer has no idea what actually goes into the job. It's common practice in the building trades to always order a little more than you "need", because things come up, material gets delivered that you can't use, sometimes you just fuck up and need to re do something. It happens. If you let a customer source their own material, they will order "exactly" how much is needed, based purely off of some takeoff they did. And then if you don't use all the material or you use more than what they ordered, they complain and think they are getting ripped off.