These games take too long largely due to incompetence. Like Rockstar and GTA, they've been using the same engine for every game since the N64 days, just improving the visuals. The problem is they re-work the concept and unique "hook" of each game repeatedly for a year or two before getting into true production. It's a terrible waste, and in the end the games aren't really dramatically different from their predecessor despite new "art styles", most of which people don't like anyway. Skyward Sword is Wind Waker in the Sky, Twilight Princess is an attempt at OoT, and all of the games generally re-use characters, music for various scenarios, etc.
Zelda could learn a lot from GTA. The fans are passionate, and also set in their ways. The Dev team should maintain a constant artstyle and basic play mechanic, but add fun new locales, cool new weapons, an interesting plot, some great new soundtrack elements and improve the visuals with each new iteration. The defining uniqueness of any title should be focused on the plot, such as the Triforce in LoZ, the origin story and dark world sealing in ALttP or OoT's time travel. Beyond that titles should focus on just being bigger and better than before.
