Perhaps the developers were thinking about expanding the functionalities of the site in a way that server-side rendering can't help them, so they started doing

Perhaps the developers were thinking about expanding the functionalities of the site in a way that server-side rendering can't help them, so they started doing it all in JS anyway. In the future, if the site survives and gets bigger, they will surely think about corner use-cases like yours and work some time in server-side rendering.