I'm a bit surprised by this. I've had mine since April 27th, but I thought supply would have definitely caught up by now. Maybe in June? I remember reading about the MacBook being completely in stock at Apple stores by the end of May.
I think the situation is slowly improving - supplies to third parties are certainly a lot better than they were. However I think Apple themselves are still short on stock and having to continue holding orders for a month before shipping them out. As for why? Constrained supply of the display and SOC would be my guesses.
I ordered my SG 512gb version 2 weeks ago and was told it would take 3-5 weeks for delivery. i walked into an Apple store on Tuesday and walked out with the exact model I'd ordered. The salesman told me that they had a ton in inventory and no one was buying them because nobody knew they had them.
Might be worth a phone call to your local Apple store or Best Buy.