The issue is when the placeholder text disappears as soon as you start typing, you can easily forget which field that was – especially if you’ve filled out a few fields.
Shopify’s default behavior is fine because once you start typing in a form field, the placeholder text becomes a label above the text you’re typing. Example – I typed “Matt” in the first name field. Compare that with the Last Name field which has no text in it yet. The idea is to not lose context (knowing at all times what the field that you’re typing in is).