you know PA has a cap, right? and you don't even need all tiers maxed, at a certain point (depending on calling) the additional rewards are minimal anyway.
what about the case that your main isn't completely geared?
the point was the whole game is focused on character, not on account. most things you do are for a single character only. to pick up the example from the beginning, why is my XP when I level not awarded to other characters as well? "the xp system keeps me from playing an alt" - sounds familiar?
achievements are not account based for the same reason the rest of the systems aren't - to keep people p(l)ay longer. do I want to do it all over again to get a specific title on another character? definitely not. do I want to level him to 50 first, and then gear him to be an alternative to my main? definitely not.
another thing anuhart already mentioned: the current system is balanced/designed for one character. IF it would be changed to be accountwide, progress/rewards would change as well (so in the in the end it might be even slower than parking a char and use his rested xp - so alts may profit from it, which does not mean much when the whole progress has been nerfed).
don't get me wrong, I'm definitely for accountbased systems (because in the end it's still ME playing), but given the foundations rift is build on... and the work/money it would take to change the system... designing/balancing a rewarding system per character is hard enough, now imagine that in the scope of a whole account. not saying it can't be done, but it will be difficult (and I'm afraid time will be an even bigger part of the equation as before then - ironic in the light why we have this discussion).