I think it's used because "hunting" can mean "searching for".
I've hunted everywhere in the house but I can't find my favourite belt.
He hunted high and low for the perfect suit for his wedding but he's going to end up wearing jeans and a T-shirt.
I'm going to hunt for the last few raspberries of the year so I can make a fruit crumble.
With mushrooms, there is also the added danger that some mushrooms are very dangerous so you "hunt" for the safe ones, and avoid the poisonous ones.
Here in the south of England, I often see posters advertising "A Fungi Forage" - that sounds much more fun than "Mushroom Hunt"!
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