I have a slightly different take on Christian’s question (Correct me if I’m wrong Christian)
What I took the question in part to be is: Why are there so many UP players who also decide they want to meddle in pipe and reed making. As opposed to GHB players who are virtually ALL content to buy pipes, and the majority also tend to buy reeds.
Whether or not that’s the question, that’s the one I’ll answer as there are lots of responses to the other one 
Why do UPers tend to want to make pipes and care about their construction? Well I think because to some extent a lot of UPers tend to be tinkering types by nature. You really have to be - and maybe somewhat moreso than with GHBs. Also, the availability of pipes is such that many people feel frustrated that they’ll have to wait so long for pipes. They just think “why not build my own”. Also, UP building is arguably more complicated and with less standardization than the GHB so chosing a maker may be a more delicate task due to things like: cost, wait times, and playability of the resulting instrument. I’m guessing if you look at serious GHB players who are buying the sets that are made by the premier makers, who have to wait for a while and pay UP level prices for instruments - they probably take as much care and interest in construction as UP players do.
Without getting too much on my soapbox, I think the itch to homebuild is the downfall of many would-be pipers who really might have potential to play a good instrument well, but try pipemaking when they really have no command of the instrument itself. My opinion (and blast away if you disagree) is that you shouldn’t really attempt to build one until you are skilled enough at playing it to recognize a good thing. The caveats to this being a skilled craftsman/instrument builder who works with a skilled piper, taking input on the design - working as a team to build an excellent instrument or kit instruments such as the Penny Chanter - and even then, letting David make it for you is more of a sure thing.
As far as reedmaking, the first argument about UPers being tinkering types still applies. Also here though, there is a lack of standardization among the UP sets out there so reeds are really a custom job. That means if you can become skilled at making them yourself, you dont’ need to send off your chanter for re-reeding, and you can tailor a reed to suit not only your chanter, but also your personal preferences. That being said, I still believe that until a player reaches some reasonable level of proficiency, he or she should leave reedmaking to those who can recognize when the reed/chanter combination is performing correctly.
In summary - pipers like to tinker, and the individualization of the instrument lends it self to customization, and to those who like to bore, scrape, sand and ream. That being said, those who are just beginning should tread with caution and realize that if their goal is to play, they would almost surely be best served by getting a good chanter made by a skilled pipemaker, with a solid reed by an excellent reedmaker, and practicing for years before even thinking about doing anything more than routine maintenance.
This is my opinion only and I recognize that there will be many people who will disagree. This has worked well for me for 8 years though, and I’d say I’ve become reasonably skilled at playing in that time.
Cheers,
Pelham