And a lot of the firms that use it also tend to be large and have very deep pockets. You pretty much have to cater to their every need, because they pay well.
It is much easier and less expensive to pay a developer to keep writing those workarounds than to recertify all of the internal applications to a modern version of IE. Furthermore there is a significant time cost to the testing and deployment of a new browser to all machines. These two times and costs are often several times if not an order of magnitude or more the developer cost.
You're applying logic to a software issue at BigCorp, that assumption is, sadly, wrong.
A lot of the time it's some kind of support contract though. Somebody at some point in time had a brain fart and signed a contract for a Website running in IE9 for the next 20 years and now they will have to support that, cost it what it might.
