Apparently not, because many people buy glass back phones that are both thicker and heavier than a good synthetic, Then they add a cover to protect the back, making the phone even bulkier.

The smartphone market is goddam moronic in some aspects IMO. It all started when idiotic reviewers claimed the Galaxy S2 that was superior to the iPhone at the time, didn't have the same "premium" feel of the iPhone. In part because it was lighter!!!
Also the Galaxy was way way more durable, the iPhone cracked if you looked at it wrong. So there is no doubt which phone was made from the better materials as in the more functional ones.
But after that "premium" phones were basically all glass despite all the downsides, like adding thickness and weight, and being way less durable, and also glass being more slippery so risk of dropping it is increased too. Unless of course you use a cover, which then again makes the glass back utterly pointless.
There is simply no way glass is a suitable material for the back of a phone. Except moronic reviewers continue to claim so, and phone makers keep making high end phones almost exclusively with glass back.