It has a health crystal and a regen bead in the recipe, and collecting all the components without running out of item slots is difficult on a non-support. Junglers (and to a lesser extent, top laners) would like to build aegis sometimes as well, and I see no good reason for it not to build out of crystalline bracer.
Thanks for reading!
AS OF AUG 7 2015 THEY LISTENED TO ME AND I AM GOD
Because Riot wants the build path to be bad since the final item is so good. Basically the same thing they've done to Frozen Mallet, but to a lesser extent. It's an interesting way to lower the value of item without directly raising its cost.
Good idea!
I agree, it needs larger items to build it into.
Because it's not a fully built item. Aside from BoRK and Gunblade, there are no fourth tier items. Anything that builds out of something else has only one more step in its build path.
This was mentioned in a discussion a while back. The reason its build path is deliberately convoluted is because 
is an incredibly good anti-mage item, due to how it mitigates poke really well, and if purchased too soon, too often, it could shut down AoE mages once teams start grouping together. A buff to its recipe would be a buff to anti-magic tank itemization as a whole, and that's not something Riot feels comfortable doing at the moment.
Riot explained this several months ago during preseason, but basically it can't build out of bracer because it is already an incredibly strong and popular item, and making it that much easier to build would require them to nerf it substantially. Most people don't factor in build path when considering item strength, but in situations like this (where it is already the premiere item for supports and junglers to get vs AP on top of having excellent gold and slot efficiency) having a smoother build path is not a good thing. It is part of the same reasoning behind having BF sword cost so much and not having offensive items build out of each other in tiers like defensive items have been changed to.
One of the most puzzling mysteries of our time.
yes please ^^
they listened to me http://boards.pbe.leagueoflegends.com/en/c/champions-gameplay-feedback/yMtrbjGs-core-defense-items-pass-516 i am GOD
