They used to be in some applications (like really big photocopiers). But toner is messy nasty stuff to deal with. Maybe refillable at a shop could work though.
I've been refilling the toner cartridges for my low-end Brother printer with generic toner off Amazon for years. It's not that bad and costs a fraction of the price of even a generic replacement cartridge. The toner comes in a bottle and you slowly pour it into the cartridge, perhaps with the assistance of a small funnel if necessary. I wear gloves and do it outside or in the garage. It only gets messy if you try to do it inside or have the dropsies. Obviously don't breathe it in.