Because, at the time, those beliefs were not universal, and a compromise was made as formation of the Republic would likely not have happened otherwise.
All the writings of the Founding Fathers are free and publicly available. You can learn all about it.
That might explain the document, but that doesn’t explain some of them keeping slaves themselves. Jefferson even went on to be president and while he did things to restrict the trade, he kept slaves and opposed some measure to restrict slavery (for reasons much as you describe).
Like any subject, I’d you want a respectable understanding it’s going to take some commitment to studying key documents, not just reading. A few quotes.
You don’t get smarter reading a few 140 characters tweets.
All the writings of the Founding Fathers are free and publicly available. You can learn all about it.