This is because useBreakpoint leads consumers to inadvertently couple themselves to the current set of breakpoints, making it risky for us to introduce new breakpoints.
For example, you may have chosen to detect large screens by checking that the user is on the (current) largest breakpoint (e.g. const isDesktop = useBreakpoint() === "desktop"), but this logic would break if we introduced an even larger breakpoint and the Hook started returning other values.
To maintain backwards compatibility, useBreakpoint will continue to return "desktop" when the user is technically on larger breakpoints.