It comes from people being cheap. If someone is only willing to spend $20 on their deadbolt, than there are plenty of companies willing to produce locks to that price point.

With that said, there are also plenty of manufacturers that produce higher quality locks. These companies seek certification under ANSI, BHMA, and UL437 standards to insure that their locks will do what they are supposed to. This is why you pay $20 for a Schlage and $200 for, say, a Medeco.