The two primary reasons are that the parts are not readily available, and the repair cost exceeds the printer’s value.
Parts for small inkjet printers are not typically available because the demand is too low. In addition, there is no economic incentive for suppliers to make replacement parts for small inkjet printers, so they don’t.
The other reason is that small inkjet printers are relatively inexpensive to replace. So most people choose to replace their broken printer with a newer model.