how come you cant acquire 1.com 2.com 3.com etc but you can

[webmasterbeta]

I was just wondering why you cant acquire the domains

1.com
2.com
3.com
4.com
5.com
6.com
7.com
8.com
9.com

also the same with a.com?
b-z.com etc...

how come these domains are impossible to buy? like I mean they were never made available? and their are no sites parked on them I dont get it?

thanks