Don't have to. You asked about Apple creating proprietary designs. I gave you the reason they do such things: it ensures stability because Apple created the configuration, and therefore they only have to worry about their own designs and devices.
I think Halfmanhalfamazing is referring to the non standard power cord.
In alot of cases that's not the case.
For example, there are some macs that will work with a standard ATX power supply. All you need is one or two barrel connectors to re-work the plug/wires.
Dell is also very proprietary. And in their case it's the same. Stability has very little to do with the equasion. It's more about trapping the customer.