Civ7 didn't copy anything from Humankind, civ switching was developed independently and overall gameplay differ by a lot.
The reason not to keep civilization through ages is based on Civ7 concepts. One of the biggest reasons is what Firaxis wanted to remove the discrepancy between "early" and "late" civs, which was a problem for all previous civ games since civilizations got any difference. So, each civilization has bonuses, infrastructure and units, which are actual only for their age. In this logic keeping civs is impossible, because Firaxis would need to develop 3 versions of each civilization to let them go through ages. They actually did it with India and China, so if, for some reason, you want to keep your civ, India and China are totally available, but making this for each geographic civ would multiply the amount of work too much.