and installing mods is so easy, why not just gather up the ones you really like, and install them yourself.
That's probably fine if you just want a few mods that each have little overlap, no? I would imagine you need some kind of organization that a modpack provided if you want multiple mods to integrate well with one another, specifically in how they're unlocked via the tech tree, not to mention consistent balance and production cost/means.
I currently am playing a game with 34 mods running, with no conflicts, just have to load them in the correct order, which EDB makes very easy to do. And the order is only important for 4 of the 34.
UOM was far from consistent on production costs, or balance.
Many of the active modders work with the other modders so that their mods will work together.
Now there are just some mods that change the game so much, they will never be compatible with anything else.