If we use basic math, we could come to the conclusion that missing the worst days would result in better returns than participating in the best days.
Why? Because after a loss, you require a larger gain to get back to even. This is an extreme example just to make the numbers easier to comprehend, but if the market fell 50% in a single day, you would need a 100% recovery to get back to even.
However, here is the main issue. To miss the worst days, you need to be timing the market. To participate in the best days, you need to do nothing except stay invested.
It is infinitely easier to participate in the best days than it is to properly time and miss the worst days. You'll often end up missing out on some of the best performing ones in an attempt to avoid the large losing ones.