It indeed has no MLG doors, covering the tyres. This is done for cooling reasons as far as I know. Because the 737 is a short-haul aircraft with many landings a day. To leave the wheels uncovered, some airflow is created along the brakes. I can assure you they're hot after a landing. I once did a brake change during a turnaround (turnaround time 1 hour). Weneeded towels to protect ourselves against the heat.