Very likely it's disabled to improve database performance. That's the short answer.
The long answer is:
Every time you load a page, the forum page talks to the database, which is called a query. It asks it who you are and what you can see. If you can see information on a page, it has to grab that, and it's not all done in one go. There are lots of checks to make sure that, maybe you can see topic names but not the posters, or categories but not one board inside it, or maybe you can't see anything at all. There are also checks in case you are an admin or moderator (who have special powers and see things we peons don't) and if there are any other special features afforded to your user.
Then the database has to feed all this into the page. It takes a fair number of queries (sometimes 15-30 queries for a page). Some are quick and easy, like pulling down information on users. These are itemized and indexed, which means they're quick to grab. Others are more lengthy because of the way they're stored, like posts. Posts are hard because they aren't stored specifically with the topic they're posted in, there's not one place you can go in the database and see all the posts in this thread and only the posts in this thread. You have to know how to find them and do so carefully. It takes the database a bit of time to retrieve all this. (Admittedly, this is all happening before you can blink, but it's still a fair amount of time in computer terms.)
Now imagine adding in the list of people who are looking at this. That not only means you have to grab that information from the database, but it has to continually update who is doing what at all times. At the moment it only does this when you look at someone's profile. That's it, when you pull up someone's profile, the database is literally going to your user, checking if it's logged in, and seeing what you're doing. Otherwise, it doesn't care. So by adding this feature, the database is going to have to ask your user, every X seconds, what its doing. Tthat takes a lot of work, and decreases performance.
Also consider that it's doing all this work for someone else at the exact same time. A lot of someone elses. In fact, everyone who is loading a page at the same time as you, which could be hundreds of people. That's not just logged in users, but guests, search engine spiders, archive.org, and other Internet site crawlers. And the database is serving all these people/bots at the same time.
The database deserves a medal.