I see your point but I think you might not know all the reasons for defederation. There is nothing wrong with wanting to interact with people on other platforms of course.

However, Meta is a huge company and it is not in Meta's interest to have an open fediverse with many diverse platforms. Platforms like instagram are notoriously predatory walled gardens. They grow until there is tons of people on them and they have a quasi-monopoly, then they crank up ads, force people to make an account and/or download an app to see content. Their content cannot be seen from elsewhere.

If their services have been closed off in walled gardens until now, why would they suddenly shift and want to support ActivityPub? Mastodon is big but not big enough that people feel they are missing out by being on other platforms. I doubt they expect to attract significantly more users that way. They want to create a way to become part of the fediverse through their platform. Given the sheer amount of money Meta has, they will then make Threads the most bestest and easiest way to do microblogging on the fediverse. Find a mastodon instance? ugh what a hassle, just join Threads. Then they can start adding features that mastodon and firefish don't have. People will switch to threads for these features, and voila, the age old strategy of embrace-extend-extinguish is done.

Even if we assume that is not their motive, the fediverse is about open, democratic and collaborative social media. Those values are directly opposed by Meta's entire business model (and their business itself which is generating shareholder profits). Now if some small company was part of the fediverse who cares, but Meta is a huge behemoth and IMO we're better off building a world without them, rather than inviting them into it to compete against largely volunteer-built software. Let's learn from the past.