Perhaps they want to build up stocks and what they have competes with what NV are bringing out next such as 20GB 3080's so by waiting they're not showing their hand to NV too early.
That just seems like a very lazy, illogical excuse. Surely get the card out, get NVIDIA's share of the pre-orders. NVIDIA can't exactly release their 20GB cards next month given they've just released their 10GB card range and release it to AIBs.. NVIDIA would look really silly if they released a 16GB 3070 next month for example when their flagship 3080 or 10GB.
So AMD announce what they have, open up pre-orders, AMD will have a bigger share of the market than they would have. Then NVIDIA 6 months from now release their 20GB cards... then AMD price drop to provide even better value and more people probably jump ship to AMD given they are cheaper, console + CPU hype, providing great performance with 16GB of VRAM which is more relevant than 20GB and NVIDIA are stuck in a very difficult place
AMD aren't going to have enough stock for pre-orders or for orders regardless of if they announce in 2 weeks or 6 weeks.. I think the painting is on the wall for that and if we look at stocking levels historically from them, it'd paint the same picture re: near launch.
The reason AMD aren't doing this suggests to me they don't have the goods to go to toe to toe with the RTX 3080 and it'd be better to announce their product line closer to the 3070 benchmarks instead.
And if thats the case and AMD are just marginally beating the 2080ti, without NVIDIA's far more mature feature set (RTX, DLSS, VRSS, Freestyle, NVIDIA audio thing which takes away ambient sound) .. then its dissapointing IMO regardless of a few gigs of extra VRAM with maybe 1-3 games will take advantage of.. and even then... no point having VRAM without the speed to back it up.