Mastodon too has several client (web)apps. The Fediverse is itself a more neutral protocol with more heterogeneous platforms based on it (e.g. Lemmy, Pixelfed, etc), so Mastodon refers more to the platform (schema, UX, wider community) than the individual UI you use to access it, which just happens to usually be whatever your instance admin serves by default.
As for apps, people simply prefer instant messengers purposefully designed for their platform. There is genuinely a lot of platform-specific quirks around things like push notifications, which a non-iPhone developer not have the motivation to figure out. Besides that, a lot of these predate contemporary cross-platform technologies like Flutter.
As for apps, people simply prefer instant messengers purposefully designed for their platform. There is genuinely a lot of platform-specific quirks around things like push notifications, which a non-iPhone developer not have the motivation to figure out. Besides that, a lot of these predate contemporary cross-platform technologies like Flutter.