Why do people use zhonias in 1v5s

So they can Ryze ult out.

only in bronze my friend

It stalls for a couple seconds. They know they're dead, but if they can stall the enemy team it might give the rest of their team a chance to get in a better position or take an objective cross map.

occasionally it might buy your team a few seconds to either come and save you or escape while they wait for your Zhonya to come down. But yea, usually its a waste of a good Zhonya.