Why printers can't work with open doors?

So I have this HP Officejet L7680 and it doesn't work because the cartridge door broke and everything I put in the sensor hole to fool the machine that the door is closed, now doesn't work either. The door was removed 3 years ago and the printer has been functioning with a stick in the sensor hole. I need to use this printer and I find it ridiculous that this is stopping it. Can someone help me with my printer and also tell me why it can't work with the door open?