We are finding evidence of pre-Homo Sapiens structures. Wooden structures typically rot. “Cavemen” is a misnomer from the evidence survivorship bias.

Also notable is your claim they were less intelligent. Human intelligence hasn’t dramatically increased genetically or biologically. Human knowledge has dramatically increased since the invention of writing recently in our history. Creativity and problem solving are highly evolved in our species and we have contemporary evidence of huge ability to transmit knowledge through oral transmission and the influence of tribal tradition.