why does submit button refresh page and not just ajax page code example

Example: submit form without page refresh using ajax jquery and php

$(document).ready(function(){
		$('button').click(function(event){
			event.preventDefault();
			var	name = $('#name').val();
			var	mobile = $('#mobile').val();
			var	address = $('#address').val();
			var	city = $('#city').val();
			$.ajax({
			    type: "POST",
			    url: "upload.php",
			    data: { name:name, mobile:mobile, address:address, city:city },		    
			    dataType: "json",
			    success: function(result){
			        			    }
			});
		});
	});