Again, that's a very good point and a very good question.
It used to be that way, and in the Canadian Armed Forces, people who were injured in service were allowed to stay in a different capacity, a different military occupation, perhaps. But when they came up with universality of service, then that became a problem, because unless you meet the physical standards.... The universality of service introduced the concept of the soldier first and the trade afterwards.
You'd have to ask people from National Defence why that is so, but I think they expected that the money they would get would be spent on boots-on-the-ground sorts of things, on people who can fight. That's why the restriction is there now.
That being said, public service priority hiring was introduced a few years back, which allows the government to actually give access to government jobs to individuals, so that they may take off the uniform but they still have access to civilian National Defence jobs.