As with any game that's been out for a while, there are simply more players available at the level cap than there are at any particular level range below that. So to ensure that there would be enough players to fill groups so you wouldn't be waiting forever, SE keeps trying to encourage (or sometimes force) high level players to keep going back to low-level content. It does have the downside, however, that some of those players get frustrated and turn obnoxious or even downright mean to the low level players they're supposed to be helping. There are certainly plenty of other high-level players who are friendly and helpful, but that's a limited consolation when you just got stuck with the wrong sort.

You can put together a party yourself when you want to be less subject to pure luck regarding who you end up in a group with. Ask people in your Free Company or Linkshells to join you, set up a Party Finder listing that specifies your playstyle preferences, or you could even just try giving a /shout in a populated area like any of the main cities asking for like-minded players who want to join you.