Why are you using PNG when it’s not backwards compatible with gif? They don’t even render a small low quality gif when a browser which doesn’t support it tries to load it.
Are you seriously asking why a commonly supported 27 year old format doesn’t need a fallback, but a 2 year old format does?
When png was released, it was unsupported by the majority of browsers (and is still not supported by everything mind you) but didn’t have a fallback to a more widely adopted format. It was finalized 9 years after gif, which admittedly is a third of the gap between now and png finalization.
Fallback support isn’t needed. It never has been before, why would it suddenly be needed now? Servers are more than capable of checking the browser on request and serving a different format based on that. They’ve been capable of doing that for decades, and the effort that goes into it is virtually non existent.
