It's very difficult to believe the fact that the first skyscrapers were built in Chicago during the 1880s Почему не were

It\'s very difficult to believe the fact that the first skyscrapers were built in Chicago during the 1880s. Почему не were being built?

Ответ: потому что разницы между этими двумя временами почти нет и в данном предложении оба варианта были бы верны. Нужно смотреть по контексту.