William is not typically pronounced Willie-um; I don't actually believe you've never encountered this pronunciation; I think you're just being contrary.
Liam makes as much sense as a nn for William as Bill does.
William is nearly always pronounced Will-yum. Sometimes regional accents make it sound like WEE-yum.
Before you criticize a man, walk a mile in his shoes. That way, when you criticize him, you're a mile away and you have his shoes!
Steve Martin