In my experience, starting with wireframes can be beneficial. It helps to get early feedback and iterate quickly. Once we have a rough idea of the layout, we can better understand what functionalities are needed and document them accordingly.
But you're right, BAs sometimes act plain lazy and wait for the design team to come up with some wires before they simply "explain" the wires verbatim in their FSDs like "On clicking the button, a window slides in from the right.." Well, that's not exactly an FSD, you're basically just describing the wireframe. A true FSD wouldn't tell you where the window slides from or hell, even if its a window or not! But I guess that's demanding too much from these BAs.