The web had interoperability but it was largely ignored and then replaced by silos and walled gardens. The Web 2.0 cognoscenti made a lot of the same mistakes as the Web 1.0 cognoscenti about how systems would be used and received by "normal" people.
A lot of good ideas had terrible usability and didn't get to a real tipping point. Contrast that with social media sites (even back to MySpace) that were far more usable to the average person.
As as an example, look at RSS (any version including Atom). It is a great technological solution for the Web 2.0 proponents. They were interested in long form postings and had a lot of posts on similar subjects. Their posts had titles, some categorization, and maybe even semantic elements. Making a "feed" to syndicate those posts was a pretty useful thing both for their visibility and you and I as readers.
What RSS doesn't offer is a good way to express short form informal posts on mixed subject matter. Even posting a simple link to an interesting page or just an image isn't well expressed with most RSS generating software and not handled well by the format or feed readers. Even the word "subscribe" implies a bit of a formal social anti-pattern.
Compare that to most social media sites, even going back to MySpace. The presented users with a single content field. The UI basically asked "What's up?". There's no implied formality of a post title or categorization. It just plops on top of a stack of your posts on your "feed".
That's not to say what social media sites have done with those user feeds is necessarily good (echo chambers, engagement manipulation, etc) but they served a desire of a lot of people. I'd venture that most people don't want to sit down and make long form posts with cognitive load like titles and categories.
Large social media sites have no use for interoperability because their money is made on engagement with their site/app. The Web 2.0 visionaries spent a lot of time navel gazing and being focused on their areas of interest so they missed a lot of thing social media has implemented. We're seeing interoperability start to happen again (Fediverse etc) with people taking the informal social media models and applying Web 2.0's decentralized concepts.
It's not clear is the decentralized social media can hit a critical mass of users because multimedia, especially video, is a huge draw for social media users and is hideously expensive to host and distribute. There's also effectively zero monetization and a lot of people are trying to be professional social media users.