In the wild there is constant competition with other organisms. Different species will win out in different circumstances depending on the exact conditions. The result is that overall, any species will find a niche to survive and reproduce (or would be extinct). But when we want to cultivate particular species, we have to eliminate all that competition to guarantee that only our chosen species will survive in our particular conditions.
If you just wanted to cultivate a mushroom, any mushroom - you wouldn’t need to create sterile conditions. You could just create a fungus-friendly environment and see who popped up. That’s all that happens in the wild.
You could think of it the same way as keeping animals in captivity. In the wild, rabbits are able to find places with enough food and shelter to breed and survive. But if you want to keep rabbits in your garden then you have to eliminate the competition and predators to ensure that your particular conditions are favourable for the particular species you want to prioritise.