I agree, using a block of aluminum the size of an oil drum is wasteful. However:
1. the CNC operator need not be a machinist or engineer, it's close relative is stocking grocery shelves ("that goes there"), once you have the program each one costs the same
2. fabricating several smaller parts requires both skills every time you make one, the set-up cost is $$$
3. the customer pays for it anyway

Boffin Emeritus