You don't know that you're not a brain-in-a-vat. Solipsism is both a logically unassailable position to take, and a completely useless one, so it is taken as axiomatic that our experience of reality is a rough approximation of what is real. It's not practically unassailable, however - because of it's uselessness.
When someone appeals to solipsism, it's time to point and laugh.
When someone appeals to solipsism, it's time to point and laugh.