how every java program extends Object class by default

Because the JLS is written that way and compilers are written to implement the JLS.

I am going to hire you as a painter. There is a clause in the contract that says "If I don't tell you what color to paint a room, paint it beige".

I then say "Go paint my bedroom red, my kitchen green. Also paint the playroom and bathroom."

You know that even though I don't explicitly say what color to paint the last two rooms, you should paint it beige.

The compiler knows that if you don't explicitly state "extends <whatever>", the class should extend the Object class.