None of it makes any sense, and no one from Bioware has bothered to explain it.
Crafting is randomly divided up into professions that aren't logical, so that whatever you may want, it requires several professions to get the parts from. A shell from Armormech, but not the actual armor. Armor and Mods from a Cybertech, but no logical reason why. Enhancements from an Artificer, also for no apparent reason.
If you need a piece of armor to slot in a shell, you can't get it from the market or make it for yourself (the schematic isn't available for that particular level) - You can only buy it from a vendor (and they don't tell you where that vendor is), but not for money. Only planet and level-specific tokens that become useless the instant you have moved on to another place.
This nonsense goes on and on, until you reach lvl50, where, for some inexplicable cause, crafters are taken out of the picture entirely by the quest reward system. They don't even let you have lvl50 schematics for Armor, Mods or Enhancements. Somehow, there is gear that continues on from fifty and well beyond, but only if you participate in the Operations and Flashpoint content. Good luck if you are a raw lvl50 noob with no purple gear and no experience, because the deadly serious matter of garnering loot requires that you have both before you can pug on a HM LI WTH LOL with the twelve year olds on Fleet.
The crafting/gearing system makes me feel like a cow in a chute waiting to get branded on my backside before being ejected out into the corral. What the heck is it all for?
Hmm, I don't know what to tell you. Why is Bioware obligated to justify anything as to the games workings to us? We have the same choice no matter what, we like it we do it, we don't like it we don't do it and this of course, applies to crafting as well.
I personally have come to the realization that crafting in SWTOR is basically valueless to me and I have every gathering/crafting skill maxed on at least one character. I leveled my latest alt without any crew skills.
On the other hand, I don't begrudge anyone who enjoys the system as is and I don't look for any explanation from Bioware as to why it is the way it is. As I said, we have the same choice always. I don't find crafting fun so I don't do it anymore. Your choice is the same.