I don't believe that it does actually. Hrm, I don't think I like that.
But, I do think that the idea that AD&D is somehow lacking in high level characters isn't really supported all that well. Look at Greyhawk. There's a buttload of double digit level characters floating around in Greyhawk, even going back to the original boxed set. Never minding places like Greyhawk City, or the Vale of the Mages. I don't think it's too difficult to make claims that double digit level characters are fairly common in AD&D.
It's difficult to enumerate the population of a fictional world which isn't fully detailed. But in general, the average level of characters on Faerun is about twice that of either Oerth or Krynn. If you are a 15th level character on Krynn, you are undoubtedly among the highest level most important characters on the planet. On Faerun, a 15th level character is probably not even a regional power. He's an average local ruler or a henchmen to a more important person. The only 20+ level character I can think of in Kyrnn at the time of the War of the Lance is Ariakas, and he's phenomenally higher level than anyone else that has stats. Looking at the who's who in DL14, Ariakas just leaps out with jaw dropping stats. But on Faerun, Ariakas isn't even particularly notable and would get lost amongst the general overwhelming might. Ariakas's duel classed nature means he's got about as much XP as a 24th level character, but even if we gave him full credit for his levels (3e style) and called him 33rd level, he still would be struggling to make the top 10 on Faerun.
So, yes, there are a lot of 'double digit' characters in both Kyrnn and Earth, but double digit covers a lot of ground - from 10 to 99. The vast majority of the characters in both worlds are 15th and less. On Faerun we can be fairly sure that the number of 25th level characters generally exceeds that of the number of 20th level characters of Oerth and the number of 15th level characters of Krynn. This is not only because Faerun has far more published material, but because you generally start enumerating NPCs beginning at the top and most influential characters, and on that basis the highest level characters of Faerun are significantly higher level than Krynn and depending on which published versions you accept, also significantly higher level than those of Oerth. But even more than that on the sheer number of insanely high level characters, the movers and shakers of Faerun though having godlike power by Oerth standards (probably 10 characters are 30th level or higher) are relatively less powerful and more constrained than the Heroes of the Lance were at the end of the War of the Lance by the simple fact that there are so many potential rivals, foils, and enemies of comparable stature.
So, sure, I don't need to quibble with your characterization of Oerth having losts of 'double digit' characters. But frankly if you look at the demographics of something like Village of Hommlet and compare it to the demographics of the Haunted Halls of Eveningstar, the difference is as stark as night and day. Yes, there are a lot of 10th level characters on Oerth. On Faerun, you have to be at least scaring 20th level to be anyone.
It isn't like I'm the first person to note this. When Elimenster met with his counterparts from Oerth and Kyrnn on the pages of Dragon magazine, one of the things bemoaned by the Krynn character was just how minor the powers of Kyrnn wizards were compared to those of Faerun.