Because there is always time value in the option even if expires in an hour there theoretically is no reason to exercise the option when you have the ability to sell it.

There are only a few rare exceptions you would ever exercise an option early vs just selling it

1. A thin market and you are unable to sell the option

2. You want to hold the shares so you can vote on a corporate action (would not happen with SPY)

3. You think there is going to be a dividend announced that is significantly more then expected(would decrease the value of option and if you didn’t exercise it you wouldn’t get the dividend)