Why VS don't compile its source codes (.cs) automatically before publishing its products!? ... if they could just see the code and study it
What does compiling achieve? There are endless products to decompile c#. Even if it couldn't be directly decompiled what is to stop an attacker looking at the compiled code to work out what is going on? If the computer can understand what it has to do.. a human can.
know perfectly that the end-product website is going public!?
The source files are not supposed to go public. You cannot retrieve them from the web server by default unless you specifically enable serving content from that directory.
Further to this it should not matter if they do go public. The only way this causes a problem is if you have hard coded credentials into the code or the code has issues leading to vulnerabilities. Many major web frameworks (which the rest of a website is built on) are open source. As are several of the major web servers available.