Different shops charge rates per hour. Why not list the jobs labor time in hours

Thanks for your response. Years ago I knew someone that had some sort of manual that listed the various vehicles and the different repair jobs by the number of hours that a job should take. This mechanic would show his customer this information. Thus the customer had a sense that they were being dealt with fairly. I was wondering does this information exist and where can I find it. Also this site would be better if the labor charge was broken down in time required.