Because then you are limited by the use cases the browser vendors have thought of, instead of enabling new applications someone else thought of. I mean, why no

Because then you are limited by the use cases the browser vendors have thought of, instead of enabling new applications someone else thought of. I mean, why not also use it for controlling a LEGO robot from a Scratch app? Configuring presets of your coffee maker without needing a mobile app? Updating the firmware of your soldering iron via vendor web page? Just providing supervised access to Bluetooth LE would satisfy these all without adding custom interface for all current and all future devices we might want a web page to interact with.