The key point here is youkai. Things will be better if we can keep them away.
Reimu, the one that's beside you is that monster indeed...
Oh! Let's blow em monsters away to the milky way!
I got it now. We'll strike while the iron is still hot! Let's get on with developing the barrier sign without delay!
In other words, we need a barrier that "Conveniently blows away only youkai and welcomes only humans".
Since it's overrun with youkai, humans are scared to come here...
There is a truth in what you said, Sanae.
...Still.