If people open the capsule on its specified date, the intervening generations will lose out on potential historical information. There’s also some doubts regarding the preservation effectiveness of capsules. The conditions they’re exposed to underground, coupled with their tendency to get lost, are the main reasons for this.
That being said, there are some notable cases where time capsule discoveries were beneficial. For instance, in 2018 historians found one in the ruins of the now-demolished Thompson Middle School in Nova Scotia. The North Sydney Historical Society finally unearthed the time capsule after hearing local rumors about its existence.