Stop asking to pick someone's brain.
Instead...
Be upfront about why you want to meet with them.
Are you looking for career advice? If so say that.
Are you researching a specific industry they have expertise in? If so, say that.
Do you admire their work and want to learn more about them? If so, say that.
Are you looking for a connection to another professional or company they're connected to? If so, say that.
Clearly stating your request will allow the person to understand the purpose of your meeting and prioritize it accordingly.
It also demonstrates that you respect others' time and expertise.
People do business with people who are respectful.
They don't do business with takers.