If someone tried to just walk in to where I work, they’d be turned away at the door. Because we work on a lot of specialized proprietary projects we can’t just let any random person walk in the door unannounced. I imagine a lot of other design studios would be similar, they don’t want any random person catching a glimpse of patent-pending or proprietary product information. When we have people coming in for tours we get advance notice to take down a huge poster of one of our ongoing projects and put it away until they’re gone and make sure we don’t leave sensitive material just laying around our desks, etc.
However, if you called enough in advance interested in an open position we have available we would probably make some time to talk to you or show you around. It may be just the culture at my company, but we’d be a little put out by the presumptuousness of someone just waltzing in expecting us to drop everything for them.
Remember, if a company is hiring that probably means they have more work to do than they have employees to handle so all the employees are probably going to be swamped with work and might not necessarily have the time to meet with you.