single deck vs multi deck

Is it true that deck means how many players play against dealer at the table?
I have seen that single deck has favorable advatage over multideck.

But then will comes question.
Why u hell have to play 2 or 4 decks when u can play 1 deck with larger bet.

And when stranger comes to your table and wants to play,then leave!

Or i am missing smth?

Just some beginner question..