Comment by theGeatZhopa

because its easier to produce (e.g. no redesign of the inners to fit the tank, no extra sausage for individual tank production, no stock-keeping, etc..),

because its easy and convenient to replace the tanks for the customer without the needs of opening the device

.. there are a lot of why-reasons one can think of and which we don't know. But, the fact that its been done that way, should tell us, there are other people thought about this already and decided to do it like its done.

If you want to be able to change YMCK individually, then look for a model with individually replaceble tanks or choose a professional device with 8+ dyes. But they're expensive.