It's blocked because it costs nothing, it incurs less risk than not blocking it, and nobody cares - corporate IT is extremely busy, overworked, trying to keep up with essential, high value tasks that impact the bottom line.

More immediately, probably it's blocked because it's on some blocklist. It's on the blocklist for the same reason: Why not? Consider these scenarios:

1. You're the IT manager. It's not blocked and something on UD causes a problem - content, even malware. You may think it's valid or not, but regardless HR, legal, managers, some VP, multiple employees, IT are all dealing with it. The VP asks you: I am extremely busy doing high-return work for the company and so are all these other people, and now I and all of these people are wasting time on this useless nonsense. Why didn't you block it? Isn't this your job? - What do you respond?

2. It's blocked, you work there and want to lookup something on UD, and you ask IT to unblock it. IT manager forwards the request to your manager, noting that IT staff are very busy integrating the new acquisition, but would be happy to do whatever your people need for important projects. Your manager asks you why you contacted IT. What is your response?

3. It's blocked, you want to lookup something on UD, and you complain to your manager that it's blocked. Your manager says: If you can't find something profitable to do, you're fired. If you can't figure out to use your phone, you're fired. Do you go over your manager's head?

4. It's blocked, you work in IT and go through the blocklist - probably tens of thousands of items long - and unblock some things like UD. Your manager says (the same as #3). What do you think?

5. It's blocked and you bring up at a meeting that you think it should be unblocked. Nobody speaks. Why are they silent?

You are there to achieve the organization's goals. UD has nothing at all to do with that or anything else of enough importance or value to even distract a co-worker for a half-second, causing them to refocus twice.