No apps perform the same function in the same way. That doesn't and will never exist. Each dev is making decisions that change what and how you post or see content. Right now if you open several apps, you will see different posts, different notifications, different content in each one of them.
Discussion
Let me clarify my thought. Different email clients work differently, but perform the same base function for user - receive and sent emails. Everything else is nuances. Let's get back to Nostr. We have a pool of Twitter-like apps with basic functionality. I don't see any advantage in using different ones for the twitter-like experience, if one works well and is available on different platforms. I liked your idea that this might be a relic of the old web. But the question of the benefits of use arose too quickly.
On the email client analogy, picture apps that can see some emails and others cannot. That's the current state of Nostr clients. Even the Twitter-like ones, they all show different things. Your feed is never the same.