A foolish question, honestly.

How many open graves have you seen, where the body is just lying there for all to see? People sealed tombs out of respect for the dead and propriety, and to foil tomb robbers in some cases.

But all of the tombs would have been sealed.

If you read the account it makes it very clear that the guards sealed the stone, not that they put the stone there. This is portrayed very well in the movie Risen, which, if you haven’t seen it, I would recommend.

JW.org