Excellent question. After my first experience hitting a ball into gorse on a trip to Scotland about 10 years ago, I said, and not just to myself, "this is like hitting a ball into the water." Moreover, as you point out, it may be worse. For example, if you can see the ball, identify it, but can't practically get at it, which is easy to happen with a ball hit into gorse, you might be able to take a reasonable drop with a one-stroke penalty under the unplayable lie rule.
Off the top of my head, I'd say one reason for the difference is the distinct nature of water as compared with gorse, which is just one (albeit a very nasty one) of many different kind of plants into which when you hit a ball recovery becomes at best problematic. If you are going to turn gorse into a water-type hazard, then what other types of plant life would you apply the same rule to? (Of course, I've played any number of daily-fee courses that mark thick brush on the sides of fairways as lateral water hazards simply to encourage play to keep moving.)
I'd also want to take look at the historical evolution of the water hazard and unplayable lie and lost ball/o.b. rules. Actually, I'd rather have someone else do that and report back.