You typically would keep the tank sizes same in volume and reduce the amount of other inks. By that other inks are empty approx. in the same time with yellow.
Similar question can be asked: Why some (consumer) supply tanks are units and can only be exchanged together in one, even there's still dye left in one of the colors? Why do cartridges signaling "empty" even there's still enough in them?
You can sell more of units and save on dye/ink.
But why would a manufacturer want to keep the tank sizes equal, in the first place? I see no practical advantage.
If anything, having them different would prevent mistakes: customers cannot install a yellow tank in the place of a blue one, because it does not fit.
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.