In our case it is because in Girl Scouts, the money goes directly in the troop account, and directly benefits the girls. Most important, it is managed by the girls too. They say what it is spent on. It is girl-led, not adult led.
Our Girl Scout fall nuts & cookie sales cover all our expenses, patches, field trips, and service projects. That money even goes to helping the community... There is no, "ask mom for $20" Whatever the girls get to do, they've funded with their own girl resources...
On the other hand, my children's school is flush with cash apparently. The money goes into some mystery fund. We have a pretty mural painted around our library. We have an ice sculpture are our dedication ceremony. We have a PTA who give themselves Tiffany necklaces and gift certificates at the volunteer luncheon.

So yeah, I feel that buying cookie dough for the PTA won't directly impact my child nearly as much as Girl Scout cookies.
I give directly to my kids' classrooms. I volunteer. But if I have to pick one fundraiser for my kids to hawk, it is going to be the one that will have the most impact on them.