True, but the structure of the rules lead to certain expectations about what is to be expected in a "standard" game. If going up levels is quick in the standard rules, high level characters will be more common in games that adhere to those rules (i.e. most games).
The DM could ban all spells above level 3 if she wanted -- but that would be a game that deviated from the standard rules, and would be quite unusual.
The rules tend to produce different kinds of games, since most people try to adhere to them "as published." Nothing prevents a DM from running a "low power" campaign in 3E, but the standard rules do not encourage such campaigns. (Similarly, in the late 70s and 80s people ran "Monty Haul" 1E games; but such games ignored many of the core 1E rules regarding experience, training, etc.)