I think Paul Graham posted a very APT answer to the mentality of most Large Companies in his Essay: 

[What Happened To Yahoo!](http://www.paulgraham.com/yahoo.html)

I have experience in working for some of these "Large Companies" and even Federal Government. 

This is what I've found to be true.

Most large companies are run by their Marketing Department, as they are seen internally as being able to generate revenue and new products.

Marketing people are not necessarily qualified to design software.  This generally comes down to a PHB (Pointy Haired Boss).

 - UX Wireframes are designed by committee and then given to a designer to tweak ad infitium ad adsurdum.
 - PHB expects 3rd party tools to be embedded in their site (this is where those inline `<script></script>` tags comes from).
 - PHB expect to be able to have full control over the content because they know HTML.  This is where those TABLES comes from (generally).

Additionally there is also politics that comes into play.  The PHB Golfing buddy owns a software house and so is given the contracts to do the websites for "Large Company" even if they're almost inept.

As long as the money keeps rolling in, managment doesn't see a problem.