Great question — answering as the UphillTech team:
Woo - one I know! - often sets are temporary structures and if you slam a door it can make the whole wall wobble (this is often seen in poor-quality shows or ones from the 60's/70's). So many floor managers have just become used to ensuring that they aren't slammed by cast members - even if it's a real set just out of habit. In fact I've seen in scripts lines such as "Door slams [ensure solid door]".
Full answer by Chopper3: https://movies.stackexchange.com/a/23790 (CC BY-SA 3.0).
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