It used to damage them outside of battle in 4th Generation but then it didn't in 5th.
I thought it was a cool little effect that was more realistic and taught people to keep an antidote with them, so why remove it?
A poisoned Pokémon no longer receives damage outside of battle, and a poisoned Pokémon now glows purple in battle. The poison status will cause a Pokémon to take double damage from Hex and Venoshock. Switching a Pokémon out no longer changes the status to normal poison. Bad poison has dark purple characters instead of white.
Yes I think the same as you it is more realistic with hurt outside of battle poison, But this is just how gamefreak wanted it.
The Pokemon are most likely not doing a lot of moving, significantly decreasing the amount od poison damage taken, perhaps not enough to note.
There may be some contraption inside of the PokeBalls that prevent a Pokemon from being hurt whilst inside its PokeBall.
Perhaps Game Freak was getting complaints that that feature should be taken off. I don't know about you but I hated that, as it accounted for more of my blackouts than anyhting else.
Perhaps Game Freak wanted to mix it up.