Am I missing something? (S&P 500)

Hello, I am new to investing and I have been looking at S&P 500.

I went on a compound interest calculator site and I put in 10% to make it easy.

I put £300 a month into it and the projections show that I could be a millionaire within 35 years if I continue to put 300 in…

This seems too good to be true and I feel like I am missing something big.

(I know it’s not guaranteed as it is a stock)