Disclaimer:
I am a web designer.Design is more than just pretty pictures. Organization of information and communicating the right messages are more important than presentation. Craigslist looks like POS but it's organized to be useful.
"So why do companies stick with, pardon my language, P.O.S. websites?"
The answer to that question has a few different roots. The three most common that I've seen are:
a) As patio11 said, the website isn't a critical ingredient in the sales cycle.
b) The competition is doing a terrible job with info organization and communication. If competition communicates horribly but has pretty pictures and you have ugly site but it communicates in a way that connects with their pains, fears and desires, you're probably going to win the sale unless design is important to what they are going to deliver for you.
c) If a website is working well (even though it's hideous), making drastic changes may result in killing the formula that was working. This happens because a lot of times when designers redesign a site, they don't think/experiment through what is already working and what isn't. Business owner gets new design, launches, sees sales go down and reverts back to old hideous design. Eventually they get to a "if it ain't broken, why fix it" mentality.
Now we've done tests where we took terribly designed sites that were working well, revamped the design while keeping the same layout, content & flow, and it increased overall conversions.
The key with improving anything that is making one change at a time and letting the numbers guide you with the decision making.
With all that said, I have launched substantial redesigns of my own site countless times and reverted back to the current version. This is because incremental changes give you less and less increases over time. Sometimes a drastic redesign can give you a relatively gigantic boost. That's how we discovered our current design. But you want to be making singular changes to test impact most of time.