How come expensive watches have to be sent away?

A bettery replacement in 90% of cases is a very easy job that takes a couple of minutes and shouldn't cost much at all. However a few of the brands, especially the Big Name brands won't supply things like gaskets/seals anymmore to replace the old ones so water resistance can't be guaranteed. The outlets that sell them are restricted from doing anything but sending them back to the brand themselves. Why? So they can turn what should be a 20 or 30 quid job into a couple of hundred quid. Price gouging essentially.

Similar has happened with mechanical watches too. Parts are difficult or outright impossible to get for independent watchmakers, so you have to go back to the brand or "authorised" watchmakers, both of whom will charge like wounded bull elephants for the priviledge of a basic service(which on a time and date non chronograph movement is not a complex job). So now the perception is luxury watches require the expense of luxury servicing.