Well, depends of the will I guess, some coders/designers are just too focus on what they do and not 3rd part addons, others don't do it because they don't get paid, others are just lazy, or they have much at hand. There's many reasons
Doing yourself also depends of the software you use, I use for styling Stylizer 5, which allows me to preview css changes before going online, saves me a lot of time, for instance, something you could do on any other CSS editor in 1 hour, takes you a couple of minutes by using that software.
Skinning an entire Kunena theme for me it's a matter of minutes or hours, depends of how much has to be adjusted. I looked at that template, quite good, simple too.