Because Web 2.0 is everything and nothing all at once. Materially it is basically nothing: a melange of technologies, most not that new. But conceptually it is everything: a rethinking of the way we view web applications and data management, a re-vision of how we approach the user interface.
That’s what Web 2.0 is. A shorthand. It represents something good – a good change in the way web apps work, a good refocusing on user’s needs for functionality and data transperancy, a welcome turn of attention back to making a better web UI. Like all such terms, it will mean different things to different people at times, and venture capitalists will create a bubble around it – but what’s the harm for all that good.