Know very little about how the vB stuff is done - but is that an OFFICIAL (as in registered/ran by) vBulletin (InterNet Brands) site? Or is it one that is "affiliated" with but owned by an individual? I would expect that if it was an official site they would not still be running 3.6.12 - but the latest version.
And nobody is preventing anyone from starting up another site (in fact, xenforomods.com is available) and doing the same thing. Those that want to push to "free" add-ons could have their own site and run any contests they wanted. Most of the involved mods here are done by a few good coders - and they've been around when times were dark and continued coding add-ons during that period (most for free). Now that xenForo is taking off I don't begrudge them the ability to make some coin. Some of those "$5" ones can be a fairly involved one that the author would like to recoup some on.
I think you can see by coders like @
Chris Deeming, @
xfrocks, @
Waindigo and several others that I am not going into mentioning provide numerous "free" add-ons in the resources as well as premium content. Should they just do it for free all the time? Do you work at your job for free all the time? Some of these folks ARE coders - and they take time out of their normal daily routines to do this. I know when I worked (I'm retired now) I sometimes did computer work for folks for free - but then I also did some of payment. No different here.
Feel free to learn to code yourself and then push it out for free. And, this is NOT vb software. I guess you will next be getting into why doesn't it have a CMS already built in, or a photo gallery, etc.
You don't want to pay for it - don't use it.... or learn to code yourself and do your own custom add-on (like one person has done that has totally customized his site - and he WAS on vB, so that should indicate something).
I spent a little time looking around the site you commented about - and noticed that a lot of the "good" add-ons are a "lite" version, minus many features that their "pro" versions have.
A lot of those "little add-ons" can be done by custom template edits and a little other work - but some people are either to lazy to learn to do them or it's just something that confuses them easily (I still am working on trying to remember how to do a template mod I did but when I updated the style it blew it out of the water - that was before I learned to make child styles of the main one).
It not only looks, but is. Several of my users comment on how responsive they are compared to the vB boards they visit.