Because everyone leaves and decides either the fence should be left open ???? or someone else will close it behind them (even the last person to leave uses this as an excuse). Which of course never happens. So the builder gets to walk through the mud and put the fence back together.
There is a legend among builders of a builder who was fined $10000 by the city for leaving a fence open or not having a fence while the stucco crew was working. So based on this absurd punishment for fence management I'm constantly aggravated by these types of problems on site. The trend today is for the trades to focus on just their part of the work and if it ends up causing problems for other stages of construction, that is ok. Even better trades can fall into this narrow perspective on the project. For this reason, each contract I make has fence management as one of the terms. Perhaps it needs a cash penalty for failure to comply? Maybe then we'd see a little more cooperation with closing the gate...
Not surprisingly the muddier the yard the less likely the person who opened the gate will closed it when they leave and instead create reasons that they did not close it - like 'i thought you wanted it left open', or 'it was open yesterday', etc.