THere were a few but they got bought (eg. tweetdeck).

There are also 3rd party apps for mastodon that a lot of people like, and they try. But for many people, mimicking the parts of Twitter they value is difficult to do without proper backend support for supporting algorithms, and even then the way activitypub works it still makes it difficult to support for most developers.

Two of the key features are discovering new or related content, which is hard to do in mastodon as it needs to calculate similarity across all of the profiles and their content in order to make recommendations -- or collect data like your cell contacts to help you connect with people you already know. Most people don't want contact sharing, and indexing all of the recommended profiles, especially across federated servers is challenging.

The second is engagement based recommendations. Many social media users aren't incredibly active. They want to open the app in specific moments to quickly catch up with everything since they last opened the app. To do this well, you need to know what they've engaged with and look back at content since they last logged on and rank it based on that. People may follow 1000 people, but really care about maybe 30-40 accounts the most. Friends, family, specific journalists or famous people. Mastodon just gives you like a sample of the last 50 or so items. If you follow anyone super active, you may just get a lot of noise in those updates.

Obviously, there are times when everyone wants a linear timeline, but it depends upon their daily use.