Because a lot of people go for the perks and when their expectations aren't met, they switch to different areas.
I don't see any other reason. If there are hurdles in way of service in the "lower tier" groups, then maybe we would have heard about them. People join civil service for the power and privileges, and when those don't appear to materialize even civil service becomes undesirable.
I am not saying if this approach is right or wrong; this is just my opinion.