Mainly because they don't want to admit it's a shitty port.... sometimes they ship it to another company entirely to do the port.
Or we can take Rage's example and not blame it on the port but on the fact it needs a beta driver no-one had (except ID) and then blame us for not thinking ahead that we should have installed the unheard of drivers beforehand.
Or that after installing said driver you may have issues with not being able to run other games due to the driver being incompatible.
Good job there ID, good job indeed.
If I ever purchased a game that said "this is not a shitty port" on it i'd be reluctant to buy it because they will probably be lying ... and eating kittens on their lunch hour because they are evil.
Only 3 things are certain in this life. Death, taxes and the PC will get shitty ports.
Some of us have grown to accept this.
It is understandable, the console market is far larger so, financially, it makes sense to optimise for consoles first.
I just wish they would follow the example of Batman- Arkham City. Take a bit longer to improve the PC version before shipping it out and customers will thank you for it (if it's any good that is).
Less headaches of rage for us and more money in the pocket for them. It's a win win.
I don't mind waiting a couple of extra months ... be happier if Batman was coming out before Skyrim but hey ho we can't have everything.
If i'm going to answer seriously it would be like this.
From a business sense it makes sense. As I said, console market is a lot bigger than pure PC gamer market so financially it's a no brainer to go for the console first then port to PC. PC games are easier to patch quickly than consoles so issues can be dealt with as they arise (and some companies are pretty quick with their hot fixes).
It's just easier and more financially rewarding for a company to optimise for console first then port to PC.
All of us PC gamers would love to get games that have had some care taken with porting them over (hopefully like Batman, as an example) but deadlines to meet and money to be made.
I'm not saying it's right from a consumers view point, there are times I get frustrated and work on my own fixes (sometimes they work well othertimes they work as quick fix until a proper patch comes out. I'm not a developer only a modder so my knowledge on fixing issues is mainly trial and error).