People from further East, had penetrated this wild zone and were "dong their thing".
The "mushroom" was a natural object. There was no body or legs at the time. That awaited the time far in the future when the Egyptians would dig down into the sand, cut block and b build pyramids.
They simply left the mushroom intact until they cut the face into it.
It's kind of like how the Moslems took over Saint Sophia. They left the building but added minarets.
Although Egypt is old, it's only the first nation state. Other places founded city states earlier.
It's easy enough to imagine amanita muscaria growing in the highlands of North Africa BEFORE the Sahara formed a mere 7,500 years ago, and then disappearing when the place dried up. Mushrooms need big, juicy trees to eat. They suck sugar from their roots. No trees, no mushrooms. No mushrooms, no soma.
Ancient Egypt was formed AFTER the creation of the Sahara. We have different peoples at different times living along the Nile.