In my experience, At large companies that hire a lot of contractors, it is not that hard to pull this off. I've seen where the contractors A team do all the interviewing, then you get a C or C- team assigned to work in your project. By the time you "give them a chance", complain up the management chain, go sideways to HR and actually change the team, the contracting firm already got 6 months worth of salary from the team.
In short, they do it because it is profitable.
PS.. To add insult to injury, the "engineers" on the team will update ther CV's to show that they worked for "large company X".
PS.. To add insult to injury, the "engineers" on the team will update ther CV's to show that they worked for "large company X".