The Knights of Myth Drannor are just one of the 'Real Player Character' groups in the Realms, and I agree that in published Realmslore to date, their levels do seem low. None of the direct lore has been written by me (John Nephew did the original FR7 writeups, using some original character sheets), and none of it has been in products that tried to tell you all about the Knights...in effect, all you've seen are brief 'highlights,' perhaps as modern-day folks might put on their resumes.
The truth is that the Knights have accomplished what many low-level groups can in a 'roleplaying first' campaign: feats that seem beyond their capabilities. How? Well, first, some characters have retired and been replaced by others; you're not seeing a stable, small band of heroes doing "all that." Second, the Knights have manipulated others into helping them, and been forced or manipulated into helping others...and these others have included Khelben, Laeral, Elminster, Storm, The Simbul, Sylune, Dove, and many senior Harpers (with Dove becoming a permanent member). With help like that, you or I could probably roll up an impressive list of victories. Third, don't believe all you read: in order to increase their influence (particularly with Florin serving as envoy, and speaking for Mourngrym -- who, like Dove, Storm, et al, is a DM-run NPC) the Knights have 'taken credit' for some work that has largely been done by others. Harpers like to keep low profiles and let others take credit; it helps to keep them personally effective and focus Zhent (or Cult, or Thayan, or Sembian merchant) anger/avenging plots elsewhere. In addition, many deities meddled with unwitting agents during the Time of Troubles; a lot of what you read of everyone's heroics in the Avatar books and elsewhere can be considered to be 'mortals unwittingly augmented/manipulated by desperate deities.'
In short, the Knights were dragged into a lot of their heroics by Elminster (not "take care of this, will ye?" but rather by manipulation), and in turn hauled a lot of powerful allies into the picture whenever they were in over their heads. They have become tireless workers-of-intrigue, and aren't above running away when they encounter foes they can't handle. Any energetic group of adventurers in a 'roleplaying first' campaign can accomplish a lot if they learn fast, get their share of luck, and plunge right in to the adventuring. Someday I'd like to tell all of you more of the tales of the Knights...and I'll be happy to answer specific queries about them, or anything else about the Realms, when I can snatch time enough. As long as we're all having fun, it's worth all the work....