Why are most name brand disposable razors cheaper than the blade replacements on reusable razors?

I own both a Schick Quattro and a Gillette Mach 3 that require blade replacements. However, it is cheaper to purchase Schick Quattro disposable razors than buying new blades for ether of my two reusable razors. There is more plastic on the disposable razors than blade replacements. How could this be cost efficient for the manufactures?