why is the oil drain plug not designed at the lowest point of the oil pan?
 

resulting in the inability to completely drain the old oil during an oil change

Because if you scrape, you want your drain plug to not be ripped off.
If you angle the car properly, you will be able to drain most of the old oil.
In case you absolutely want to get the last drop of the old oil out, you can use a syringe, but normalls this is not necessary.