Inkjet cartridges include more than just ink. At the bottom of the cartridge is the printhead. Every time you change ink, you put a brand new printhead on the machine. The heads are designed to last for exactly the amount of ink in the cartridge. It's a wasteful system for sure, but robust printheads that will last through many gallons of ink are absolutely huge (I use one at work; it takes ten minutes to prime itself on startup).
Do not buy refill kits or recycled (they're simply refilled, really) ink cartridges. If you hate paying for ink all the time, buy a laser printer. Toner is an order of magnitude cheaper per page.