Probably because Europeans either made little worth attributing to aliens OR in cases where they did (Rome, Greece, later cathedrals, etc) lots of contemporary

Probably because Europeans either made little worth attributing to aliens OR in cases where they did (Rome, Greece, later cathedrals, etc) lots of contemporary documentation exists today. In cases where an impressive structure in Europe was created but for which no contemporary documentation survives, aliens are indeed given credit. Stonehenge comes to mind.