Teams want to do whatever they can do to get as much info as possible so if they can get guys in their gym and work with them they are going to do it. This gives the coaching staff and anyone else one additional shot at looking at these guys. Nobody is going to turn down that chance. Q if you were running an organization would you not do these workouts at your facility if the opportunity was there?
As for not taking guys word on how good a player is of course you take it with a grain of salt but also people aren't going to straight up lie to you especially some of these trainers. They have a reputation that's probably reasonably important to not ruin. I don't know what some of these trainers get paid but it's probably pretty good money and if they do well they may get hired by a team and even work their way up to being an assistant coach at some point. These guys are gonna be biased and that should absolutely be accounted for because they work with the player and some of this stuff is just workouts but the idea these guys are gonna feed you some BS doesn't make overall practical sense.
Why don't guys workout? Because some teams in the past have seemed to make a decision based on a great or bad workout. If you think you are in good shape where you think you will be drafted then why work out? Should they be more competitive? Yeah sure probably but this is a business and their agents tell them to do one thing are they supposed to go against the people they are paying to help them? These are kids seriously kids. Ok they are not kids but sometimes I think we do forget how young these guys are.