Why Windows cannot call WndProc() directly?

[joscollin]

Why Windows not calling the WndProc() directly instead of pushing the message to the message queue and call WndProc after getting the message back from the message loop.

[Myself dot NET]

One reason is so that the thread that created the window processes the message. I'm sure other reasons include the fact that a message queue system is more flexible than directly calling window procedures.