What is wrong with using DateTime.Now. as main part of Unique ID?

I used to use RNGCryptoServiceProvider to generate string-based Order ID's, but, there were 4 instances where ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789!@$%^*()_- would generate an already-existing Order ID.

And it got me thinking...

Why can't we just use something like:

<html>
...
<p>@GenerateOrderId()</p>
...
</html>

and:

public string GenerateOrderId()
{
   return "OID" + 
      DateTime.Now.Year + DateTime.Now.Month +
      DateTime.Now.Day + 
      DateTime.Now.Hour + 
      DateTime.Now.Minute + 
      DateTime.Now.Second + 
      DateTime.Now.Millisecond;
}

I've been told that it's a bad idea, but without any explanation, or reason, it's still stuck in my head. ... The reason I'd like to use this, is because it will always be unique.