They actually observed the natives setting those controlled burns, and decided to themselves, “Yeah they’re wild savages, they clearly don’t give a fuck about anything or know what they’re doing, here’s me with syphilis and muskets and I think it’s time to share civilization with them, they’ll thank me later.” All that magic prairie ecosystem (which is basically gone now) was a carefully constructed environment maintained over generations to make hunting big game cheap and easy. But no, let’s have railroads and lead paint instead.