Probably for the same reason that millionaires are not allowed to purchase M1 Abrams tanks even if they have the money? Even if you can produce something for free, it isn't always in your best interest to make it freely available for simple safety's sake. I doubt Union would allow you to freely print monster trucks either, you'd at least need to prove you can operate one without killing people. From a military perspective, it doesn't make sense for everyone to have a mech when they're specialist vehicles that take a lot of training to use, and while they may be free for the players, they're almost certainly not free for Union. Free food and free housing does not translate to free mechsuits. Fighter pilots don't have to pay for new jets when they bail out, same probably applies to Lancers.