Is it?
Nothing, it's just a bit too ambitious.
Let's look at it from a working persons' standpoint - generally, people with the skills required to make something like this already work full-time, and would probably burn out if they'd get home only to work more, not to mention social life and family responsibilities.
For example, I write C++ for a living. After 8 hours of work, it's really hard to focus on another project once I get home. Hell, it's hard to focus even on smaller hobby projects. Look at this crappy raycaster:
That took five months, and it's just barely 5,000 lines of code. Now imagine making something as complex as Warcraft III engine and tools.
You'd think throwing more people at a project would make things go faster, but it doesn't really work that way, especially when you're dealing with a project where everyone works for free on their own volition. Also, the more people you have, the greater organisational effort is required. Issue tracking software, collaboration boards, version control systems, and so on. Things add up, and suddenly it's not a hobby project anymore and becomes work instead.
There are projects which have successfully pulled this off (Battle for Wesnoth, for instance), but they are much smaller in scale and more self-contained than what is proposed in this thread. Also, they are spearheaded by actual developers who bring substantial contribution to the table rather than just ideas. In the end, ideas are worthless if there's no implementation.
Don't get me wrong, if you want to make this happen, by all means - do it. I just think it's a bit unrealistic to expect that someone will do it for you. If you want to see some change, take point.