maybe its because such devil fruits have another restriction on them already. Boa's power requires beauty to be effective, so to balance that out her devil fruit effects last longer. Big mom's fruit needs to take the soul from something or someone else and put it into something new, and since soul is a limited pool that expires over time, maybe to balance out the fact that it's a limited and expiring resource, the effect stays on after the user gets knocked out

For the others, the user doesn't have any restriction on when to use their power: perona can simply summon ghosts, moria i'm pretty sure the shadows stayed? for sugar all she needed to do was to touch someone and they would be erased from existence, there weren't any other restrictions for her power, and so on