First of all, the zones are occupied by the ultimate bad guys and they're everywhere. To say the zone is densely packed with evil is an understatement. It's hard to avoid fighting as you work your way around...ALL the time. Secondly, the architecture is cleverly broken, interesting, and arrayed in such a way as to create some serious barriers, forcing you to engage rather than skulk about. Yes, skulking is sometimes possible, but often not. Thirdly, the elites (not champions, just, lol just, elites) roaming about tend to be a PITA, in part because of adds and in part because they have very annoying spells or super hard hits.
The MegaServer SolutionIn short, once you hit the last several zones, you fight. A lot. However, to balance the difficulty a tad, ARENANET has done something pretty interesting. It's fairly recent, but it's both good and bad. Or rather has an upside and a downside. As in most MMOs we had our own "home" server, though it was easy enough to party with friends on other servers by going through Lion's Arch city, which originally was so popular it required an "overflow" server. That is, you and your buddy from the other server would go to Lion's Arch and end up tossed onto a separate server referred to as overflow. At some point your home server's access to Lion's Arch would come available and you'd get asked if you wanted to travel back to your server. Anything you did on the overflow server still counted and you could be with friends without them having to transfer to your server. Pretty sweet, albeit limited.
A couple of months ago, ARENANET went whole hog with the megaserver solution. Now each zone within the world is a megaserver. The upside is, you're rarely alone in a zone. As noted above, having buddies, even PUG buddies, as you move through the difficult upper level zones is a real boon. The only downside is for people like me who deliberately chose an RP server, in my case an unofficial RP server, because it has nicer chat (none of that WoW Barrens middle school chat). RP servers have a sense of decorum, and you don't see people named (real player names here): Nips McChesty or Ulga SteelMellons. =sigh=