I agree that it's an incredibly inefficient way of training people. And its so frustrating when coworkers withhold vital information from you. Been there, done that. Companies do that because they don't have anything written down and don't invest the time and effort to create a policy and procedures manual for all functions in the company. Nor do they hire someone to do that if they don't have time to do it themselves. Many companies just get through the day. It's a wonder they stay in business.

Ask your supervisor if there are any written procedures.

I've been in the same situation all my working life. The only thing I can think of to do is to work for a better company. I'm going back to school now to improve my job skills to make me more attractive to better employers.