Why doesn't the air filter fit well enough to keep out the dirt?

A tank of fuel ago I serviced the engine, oil change, clean air filter, and grease transmission. I noticed that behind the air filter there was a large amound of dirt that had gotten around the air filter and into the carburator. I'm dissapointed that the air filter fits so poorly as to let dirt into the engine. Since then I've oiled the sealing surface of the air filter and greased and still keep getting dirt by the filter. I plan to install a brand new air filter and use alot of silicone to provide a seal but have already had a poor fitting original equipment filter fail which has caused the engine to start to burn oil and smoke. What a dissapointment.