That's the difference between conditioning and strength. Try 200lbs for 20 reps to get another view into conditioning, or pushing a sled.
A well rounded training program will address the following points:
- Skill: if you compete or are learning new exercises, you have to hone your technique
- Strength: this is well understood, you have to be strong enough for your goals
- Hypertrophy: even if you aren't aiming for physique, hypertrophy helps improve the energy available to demonstrate your strength.
- Conditioning: the ability to continue to be strong over and over and over.
- Mobility: you have to be able to have enough range of motion to perform your skill correctly.
When you start working on your conditioning, there are several approaches. At the lowest level, you need to have the conditioning required by your sport or discipline. For example, if you spar, you need to be able to last more than one round. So you train to be active for the time of the round, with rest periods that would match competition. If you simply want better wind, you can strap a heart rate monitor on, and keep your heart rate in the range between the Aerobic zone and the Anaerobic zone. But set goals and work toward them just as you have with strength.