Most people, probably including those who created the time capsules, like nice 'round' numbers for some reason. It's why we have terms like centenial, bicentenial, etc.

People bury time capsules in the hopes that it is unearthed 50 or 100 or 200 or 1000 years

You'll notice that most time capsules are buried with the expectation that it is unearthed on non-arbitrary dates.

https://en.wikipedia.org/wiki/List_of_time_capsules

> Also, why shouldn't the people who discovered it get to open it at this point?

The people who created the time capsules, if given the choice, would probably want it opened in 2028 rather than 2023.