You don't get the same quantity of ink in the cartridges.

When I used to sell computer equipment, I knew the numbers and while I don't know them specifically now, I can assure you the multi billion dollar companies didn't start putting more ink in the cartridges that come with their printers. They are starter cartridges and print only a portion of the pages replacement cartridges will print. In my day, the cartridges that came with Epson, canon, Lexmark, and HP (the brands we sold) typically had 20-30% of the capacity. Once in awhile Epson would sell a printer with full cartridges as a promo.