Because she believes it's impossible so it's impossible for her.
@ogmomo Whatever it is that you want to do, you need to align your beliefs with it first. If you want to get 8 hours of sleep, from now on your belief system should churn out that you are a person that gets 8 hours of sleep. You want to workout 6 days a week => You
are someone who works out 6 days a week. You want to eat healthy => You're a person who eats healthy.
Make your goals part of your identity through the magic of belief, perform actions that are in-line with your new beliefs, and use those actions to use as evidence to strengthen those beliefs. Rinse, repeat and in 6 months you should reach your goals.
If you workout 5 days one week and your belief system says you're a person who works out 6 days a week, ignore it. But your rational mind won't let you which is why I suggest you don't have arbitrary numbers in your goals. Instead of 6 days a week, go for a certain physique that you want or even better just be a person who works out. Make it simple so you can make it easier for your mind to believe in your new identity.