Because every product manager thinks they know better. I and other engineers have pushed back to "use the platform" for years and years and more often than not we get told to do something custom to please product's fantasies, because "the platform" doesn't implement some inane custom request. So we make something custom and worse at great expense of user experience/expectations, implementation cost, and maintainability.
Not that I'm bitter about it or anything.
The login site for my work does everything server side and serves up vanilla html that looks "from the 90s" in the context of present websites. After some slight adjustment, it's heavenly, a miracle of consistency and keyboard shortcuts.
And yeah, the thing is both designers and managers want a misable GUI that looks like a floating cloud. From their point of view, that is better. GUIs with little functionality but pointing the user at what they are supposed to do. Treating the user like a moron, not because the user inherently is one but it's convenient if they are.