Why would you NOT lean on takeoff with a Lycoming engine?
You are correct - you do want highest power.  But you also want an engine that doesn't overheat and destroy itself.  

Your initial climb-out is at an airspeed considerably less than cruising speed so you have less air flowing through the cylinder cooling fins.  To compensate for this, flying full rich keep the combustion gasses cooler during the climb-out.  

When you reach cruising altitude and level off, you may lean it out in accordance with the engine manufacturer's recommendations.

This "full rich on takeoff" procedure is not limited to Lycoming engines, either.  TCM Continental and even the older Pratt and Whitney radials also go full rich on takeoff. It is close to a universal procedure for piston powered aircraft engines.