With at least a million people playing CS around the world (including LAN play) at any given moment, "lots of people" using only a few guns is really nothing less than THE overwhelming majority.
The weapons do seem somewhat balanced when compared side by side from a performance and handling perspective, but I think the monetary aspect of the game is rarely taken into consideration.
For example, with the MP5 costing much less than the P90, it is always easily purchased by anyone desiring an SMG and thus becomes THE SMG that everyone eventually ends up mastering.
Once you've realized how effective the MP5 can be even late in the game, would you even care to switch to a more expensive SMG with only a marginal improvement in performance?
Unlikely, either because you are already confident with your mastery of the MP5 (such is the case with the UMP), or because there are more potent weapons available within your budget (M4/AK or one of the new cheap ARs.) The same case can be applied for the AWP versus the G3.
Basically, it all boils down to simple cost-effectiveness. The more expensive weapons in a given weapon class should be better than the cheaper ones. "Life is not about the second chances. It's about the little mouse, and his voyage to an exciting new land. That, my friend, is what life is."
"Nothing livens up a robotic hymn of doom more than an amazing pair of jugs." - Brak