Because such answer(s) do not solve the problem. The problem is still there. The next person with the same problem who search/find the thread do not get his answers/solutions.

Lets play with the idea of the 'use X,Y,Z instead' response. You give a person a reply e.g. to use XFCE instead of Gnome. All is well until he runs into a different problem with XFCE. Now he asks again on the forum what to do, but he'll be met with the same reply as he had with Gnome; 'Use another DE, it will solve all your problems!'.