Because the odds of Google existing in 20 years are fairly high, whereas the odds of your particular Discourse forum existing .. isn’t? 
Basically, don’t break links. It’s bad enough that shortening happens at all, which breaks huge swaths of links when a “shortener” inevitably disappears.. but the best you can do is rely on a very reliable shortener at the center of the Internet, aka. Google.
Choice is fine, but choosing to create your own “service” is a bad idea.