Your body is a direct reflection of how you eat and what you do to it. You may not be able to tell what's happening on the inside, but everyone can see what you're doing to the outside.
If you have a terrible diet:
Why do we go to the store and buy nice/cute clothes?
Why do we take the time do do our hair and/or makeup in the morning
Why do we stop to sprits ourselves with that perfume or cologne?
Heck, why do we even bother showering?
Because we want to be appealing. We want to look good.
Picture yourself walking down the street, how do you want to represent yourself? Clean, well put together? As someone who takes care of themselves and their wellbeing?
If you take the time to buy nice clothes, do your hair and makeup, or shower even, why wouldn't you take the time to eat healthy and workout?
You're doing more for your body eating nutritious foods and being physically active than you would be doing your hair and makeup every day for the rest of your life.
People can tell when you eat like crap. People notice that your body is physically inactive. You can't hide behind nice smelling perfume, makeup and cute clothes. Your health will catch up with you. Make your body thank you, don't make your body turn on you.