Why use signed integers?

I've noticed that, in quite a few MMORPGs, the stackable item/money cap is (2^31)-1, or 2,147,483,647.

But why use a signed integer? Do you really need negative amounts of items?

Why not use unsigned integers and then have the cap be (2^32)-1?

>>22
A few languages specify 31-bit integers.
True, it's not the architecture, but >>20 is just a baka. I suspect most dedicated game servers are 64-bit, and have been for quite a few years.