I believe this topic has been brought up many times before, but...

Part of the reason for the rigid lines drawn between the different jobs is to prevent any one job from becoming excluded from some/all content. This is mostly their reasoning behind the lack of diversity in the gear (having certain gear optimized for certain dungeons, etc). Another point I've seen made that seems valid to me is that in the end, the playerbase will take all the extra customization options, narrow down the best ones, and then 90% of the playerbase will simply copy/paste those, making that customization only meaningful for the trailblazers and those that can somehow deviate from the cookie-cutters without getting tossed out of a group in the process. Then you've got a bunch of other custom options the devs sank time and money into developing only for them to get thrown out, and the only way they can balance that out is to reshuffle in a "flavor of the month" system so that all the options will be valid at least some of the time.

That's just a quick rehash of what I've gleaned from the arguments so far; however, it's entirely possible there's a way to have the cake and eat it too, but it seems like SE's playing it safe for now. (Personally, I don't share in the complaint about the customization options, but I'd be the last to discourage the discussion)