Why does Level 55 PvP even have bolster? I get the 10-29, 30-54 but why would 55 need it? We're all the same level.
Because PvP, unlike PvE, has no real tiers of content to choose between based on your gneral level of gear progression. The difficulty is based on whoever you get put up against, and there's no real control over that.
In other words, because they presumably didn't want the situation we had with level 50 pvp - beginners in recruit gear existing as little other than cannon fodder for anyone who'd been playing for more than a couple of weeks and thus had massively superior gear, to the point that people in recruit gear were essentially useless to have around even against opponents in War Hero.
'Course, bolster is broken as all hell right now, so it hasn't worked out particularly well.