Required attribute doesn't work - html

Question

required attribute doesn't work on my form. What am I doing wrong?
Here is my code in HTML:
<form id="main-contact-form" method="POST">
<fieldset class="form-group">
<input id="name" placeholder="Nome" class="form-control" type="text" tabindex="1" required /></fieldset>
<fieldset class="form-group">
<input id="email" placeholder="E-mail" class="form-control" type="email" tabindex="2" required /></fieldset>
<fieldset class="form-group">
<input id="phone" placeholder="Numero di telefono" class="form-control" type="tel" tabindex="3" required /></fieldset>
<fieldset class="form-group">
<textarea id="message" class="form-control" rows="8" placeholder="Scrivi un messaggio.." tabindex="5" required /></textarea></fieldset>
<fieldset>
<button name="submit" class="btn btn-primary" id="submitBtn">Invia Messaggio</button>
</fieldset>
</form>
<div id="feedback"></div>
Here is the php code in the head tag:
<script>
$(document).ready(function(){
$( "#submitBtn" ).click(function( event ) {
//values
var name=document.getElementById('name').value;
var email=document.getElementById('email').value;
var phone=document.getElementById('phone').value;
var message=document.getElementById('message').value;
var dataString = {"name": name, "email":email, "phone": phone, "message":message}
$.ajax({
type:"post",
url:"submitForm.php",
data: dataString,
success: function(html) {
$('#feedback').html(html);
}
});
event.preventDefault();
});
});
</script>