If they can't experience their game themselves, they should at least watch people playing it at a high level.
They do? Do you really think the Senior PvP designer doesn't watch "high rated" streams?
The game isn't all about balance at a 2400+ level. The majority of the WoW arena population isn't even close to 2k. The majority are playing 2v2 and are in the 1400-1600 bracket. The game has to be fun and easy to understand (not changing abilities every week) for new players to learn and grow.
Not to mention there is an entire other aspect of the game (PvE) that has a huge player base of it's own who all have different opinions on what are good changes and what aren't. They matter too.
The bottom line is we've seen constant changing to PvP this expansion more than any other. There is effort and they are listening. The game has so many abilities and other factors to consider that the devs are just limited in the amount of changes they can push through.