You hire contractors for temporary work. Because hiring government employees is a long term investment. Government employees have great job stability because they rarely downsize like industry does. I don't like it either but the other option is to have a lot of surplus labor if projects end.

The other reason is that the government just doesn't pay enough for specialists. It's a side effect of slow change due to bureaucracy which helps prevent corruption in house. Also that pay is capped at the VP level which really needs to be raised.

Almost all of these problems could be mitigated but most civil service changes require Congress. So blame them and the Republicans desire to make government workers the enemy so no one really wants to fix it.