The mushroom spot in this game seems to be selected “only one in a certain area” from the candidate POIs managed by NIANTIC (the plot is done in the S2 cell). (Mushrooms can probably only exist in one within Level 15).
For reference, take a look at the Pokémon GO S2 Cell Viewer created by Orangkucing Lab, which will show you the areas where mushrooms can be found when Level 15 is set to Level 15.
There is a limit of one mushroom per area, so if the player density is high, the demand for mushrooms in the area will be high and the area will be “overflowing” with mushrooms.
If the area is made smaller, the number of mushrooms can be increased, but even now there is a problem: mushrooms and big flowers are too close to each other, making it difficult to tap the flowers properly. Also, Level 16 will reduce the cell size by 1/4 and increase the mushrooms up to 4 times, which I believe will create the problem of too many mushrooms.
Therefore, if you live in a place with high player density, I would suggest that you make friends with low player density to spread out your forces.
Even in Korea, the player density is not high in all areas, and some of my friends are Korean players.