One reason for the large numbers is that so when data is imported into a drawing it is immediately obvious whether it shares the same coordinate system. If multiple sets of points from different disciplines are imported into the program then the draftsman knows whether the coordinate systems are the same by visual inspection (to a rough degree).
Also because nobody wants to use negative numbers or have the possibility of a negative number showing up.
When you’re working with a couple guys who probably went on a bender last night out in the middle of nowhere with no cell service and a piece of shit walkie talkie that has spotty transmission you sometimes end up having to throw up hand signs to signal northing and easting values. There’s no hand signal for negative. So yeah, negative numbers are hard to work with in that case.
Why isn't the origin at (0, 0)? It's like saying your ground is at 1,532 volts: You're defining the coordinate system, why make work for yourself?