IMO, it’s simply because they are supposed to be different kinds of social media, the same way Twitter is different from Instagram. One is primarily for microblogging and the other is catered for images/videos. Sure you can probably create a different UI for twitter to make it look like instagram, but by trying to merge them you inadvertently dilute their core features. Pixelfed probably has more features that cater to images and videos, while Mastodon does not because it’s primarily a text platform. And Mastodon has more microblogging features that Pixelfed does not require for it to be an image sharing platform. So why would a user want to use mastodon with a more image-focused UI if they want to share photos (and not microblog) when they can just use Pixelfed that is designed for it?

I don’t think activitypub has anything to do with it, it’s just that both platforms were designed for different things. When a platform tries to be everything, it usually becomes messy. Look at short form video for example. At first it was just vine, then tiktok of which it was the primary feature. Now every social media has its own short form video (reels, shorts, etc.) because they’re trying to grab users by being an everything app, and they get bloated.