You are not taking into consideration that leverage works in both directions. With SPXL, you are assuming that it will only go UP. You also have SPXS which will give you the inverse return of 3%. So if the market goes up by 3% in one day, you would see +9% in the SPXL but if you had the SPXS then you would see -9%.
Let's say you lost 9% in one day, now you need to make that back. How much do you have to make in order to get your 9% back??? The answer is 9.89% just to get back to where you were.
Let's assume you invest $100,000. Have the markets ever gone down 3 days in a row?? Of course they have. Let's say 2% each day for 3 days the market declines. You would lose how much? Your assumption would be 2% x 3 (triple leverage) x 3 (for 3 days) = 18%, correct? Wrong! Because of daily compounding it is 16.94%.
Now, how much do you need to make in order to get that back? If you are thinking that you need to make 16.94% to get it back that is also incorrect. You actually need 20.39% just to get back to $100,000.
So to answer your question, the reason you don't want to just throw all your money into a 3xETF is because it will not behave the way you think it will. Just take a look at the returns of the ETF over it's history and it has not returned 3x the S&P return.
That said, it does return 3x the DAILY performance of the S&P. So, you need to exit the position the same day you enter it.
Let's say you lost 9% in one day, now you need to make that back. How much do you have to make in order to get your 9% back??? The answer is 9.89% just to get back to where you were.
Let's assume you invest $100,000. Have the markets ever gone down 3 days in a row?? Of course they have. Let's say 2% each day for 3 days the market declines. You would lose how much? Your assumption would be 2% x 3 (triple leverage) x 3 (for 3 days) = 18%, correct? Wrong! Because of daily compounding it is 16.94%.
Now, how much do you need to make in order to get that back? If you are thinking that you need to make 16.94% to get it back that is also incorrect. You actually need 20.39% just to get back to $100,000.
So to answer your question, the reason you don't want to just throw all your money into a 3xETF is because it will not behave the way you think it will. Just take a look at the returns of the ETF over it's history and it has not returned 3x the S&P return.
That said, it does return 3x the DAILY performance of the S&P. So, you need to exit the position the same day you enter it.