Think it’s just a familiar form factor for the product given that most are used to how the former birdsite’s apps (both first and third parties) looked and operated. A few of them are even made by the same developers who had made third party apps. May eventually drift towards something else, but at least for the time being, telling people “instead of logging into this website, you log into this website” and everything else works and looks the same is an easier sell especially during the mass adoption and scaling phase of the platform
For me, Mastodon design is unusual even after several years of using this social network. Plus, there are many apps with different UIs for Lemmy and this is not a discouraging factor for new users.