Because they haven't taken the time to put it up yet...
It's really not that hard to make a copy of your BIOS with GPUz before flashing, and if you ever want to flash it back to stock, use NVFlash.
The difference between using NVFlash & using the tool eVGA provides to flash the BIOS is literally 1 extra step (I.E. typing "nvflash64 -6 *InsertBIOSNameHere*" without the quotes your your bios.rom name is in the **s) - it's easy enough that even I, who hates messing with things like that, have done it... I went back to the stock 3090 BIOS for a while and now I once again have the "Normal" XOC BIOS on my OC BIOS switch position, because it allows 0 RPM mode on the onboard GPU fan, which I prefer because why have that fan spinning when the card is idle (at 22C no less, Mem & VRM temps are in the high 20s/low 30s at idle, with the fan OFF, so it doesn't need to be on when idle, I don't know why eVGA didn't allow 0 RPM mode on the OC BIOS of this card)