Because generally they are of poor quality, don't scale well and don't do exactly what we need. The amount of time it would take us to do a code audit to verify they were coded properly and fix the parts that are not, we could have written it ourselves from scratch exactly how we want in a shorter amount of time. If you want something done right, you need to do it yourself.
That is a completely different effort alltogether. Coding a complete forum-software, with everything working as it should, is quite a project. It would probably be possible, but would not yield a good effort to benefit. I'm assuming.
The time to do a code audit of forum software is less than building it yourself from scratch. Add-ons are typically small things that can be coded from scratch in an hour or two. Either way, I have yet to see an addon that I wanted to install that I couldn't code from scratch really quick. These are the only add-ons we have installed here:
You said that the syntax highlighter is not an add-on, but it looks like one. Also, what is digitalpoint scalability? Just curious.
The one you see there is an add-on. https://marketplace.digitalpoint.com/geshi-syntax-highlighter.990/item
I know what scalability means, but does it eliminate queries or do something like that to make it faster?
It does a whole lot of stuff. None is which applies to any person or any site outside of digitalpoint.com Unless you run your site on a whole bunch of servers you own, what it does will be of no interest to you. And since it's in-house only (not released), it's all moot anyway.