Annual expected returns

Hi
Ive been looking at the expected returns on blackjack over a year. This was based on 30 hours a week of blackjack. Now from what ive read the you annual return can be somewhere around 200%. Therefore if you were to start with 10 thousand dollars you would have 30 thousand by the end of the year? My first questions is that is this realistic? And my second questions is that if what I have read is true and an annual return of 200% is achievable why is the casinos no flood with counters because virtually no investor in any field can promise such things.
Thank you
9==9