.com, .net, .org are TLD's and not country codes.
The US does have an equivilent of the .uk domain - .us
However as with many country codes this is not divided into subdomains and if you like you can just buy a .us domain (well as long as you can qualify for one).
The .uk country code is looked after by Nominet.
Their decision was to subdomain the .uk country code.
Personally I like this as it gives us a lot more domain names to play with.
Example, my name is Mr. Smith.
I would have had a chance on smith.co.uk, smith.me.uk, smith.org.uk rather than just smith.uk
With some countries you can simply buy a top level domain, in others it is like the UK and the local governing body have decided not to allow the sale of domains in the top level name space.
Anybody from anywhere in the world can buy co.uk, me.uk, org.uk domains.
Again, Nominet's decision.
.us for example are only open to residents of the US, a lot of country codes are restricted to residents of that country.
Other countries allow open sales because their country code is interesting to others (.dj for example & .to) and so see it as a major line of income.