The creation of the internet was Web 1. That seems pretty straight forward. I don't think anyone will argue with that.
The shift to web2 was more subtle. Most people never called it web2. It was (and still is) just the internet. The difference was an entirely new set of capabilities. Users could now interact with websites. They could directly edit data that lived on a server. This became essential functionality for most websites.
Web3 could have been used to refer to other innovations. How about smart phones? Websites had to be designed responsively and the way users interact with websites changed a fair bit.
But here we are using web3 to refer to blockchain-powered decentralized applications. This is definitely a noticeable shift both for end users and website hosts. The infrastructure changes, the trust model changes, and it's an entirely new mental model.
But why is this web3? Why not web4/5/whatever? Haven't there been other major innovations?
Sure there have. Ultimately, this comes down to marketing. Because no one owns "the web" in whole, there's no one person to ask whether we've reached a major milestone or not.
Blockchain has successfully claimed the term web3 in the minds of the general public. That's it.
P.S. It's a pretty effective marketing strategy at that. There are many self-identified "web developers" in the world that probably never would have given blockchain another look if not for the term web3.