A suit is not an automatic requirement for all job interviews. A co-worker of mine said he interviewed with the IBEW to be an electrical apprentice years ago. He wore a suit and in the waiting room all of the other interviewees were wearing work clothes like you would wear on a construction site as a laborer. He said he was very specifically not chosen to be an apprentice because of his clothing. That may not have been the only reason, but he said they practically ended the interview as soon as he walked in due to the suit.
When I was asked to come in for an interview for the job I have had for 23 years now I was specifically told not to wear a suit. I wore nice dress pants and a dress shirt. Not even sure I wore a tie. Now, it is generally never appropriate to wear shorts, a sleeveless shirt, and flip flops to a job interview unless it is a very casual environment and that is what the boss and the rest of the employees wear every day. Even then I would want to wear something a bit better than that.
