Let's start at the beginning...

There's a plethora of ancient sources/records depending on which monuments we're talking about. Some say giants, some say "Annunaki/aliens", some say "gods", some say men, etc. The "by whom" is fuzzy since every civilization likes to rewrite history to make it appear "they" were the cradle of life. The "why" to me is the most intriguing part. I find it personally hard to believe that generations of workers slaved away at what mainstream would have us believe is nothing more than giant art-class sculptures. I think most if not all served an actual purpose other than aesthetics. And if you want my personal opinion, I think most are pre-flood relics, built by giants and/or fallen-angels, ie. Nephilim.