About the slavery thing, it is complicated. On the one hand, we have a few people who might feel that slavery was wrong in principle. On the other, we have a larger group who felt that their new nation was still under threat, and that unity was worth the compromise. Bearing in mind that none of them actually saw Black people as anything but benighted heathens, anyway, the deal was inevitable. Wrong, but inevitable.
In the matter of the Constitution itself, we are an abominably lazy species. It was (and is) easier to stick an extra paragraph or two in than to rewrite the whole thing from scratch. There is also the fact that , once a standing army, the National Guard and police forces had been established, the Second Amendment -which is about maintaining a militia - became obsolete, but nobody thought to remove it. That could have been done in the 19th Century, when 'gun culture' was far more prevalent in Engand than in America. If we did things the same way in the UK, all professional and amateur sports except archery would still be illegal, and every adult male would be required to own a longbow and practice weekly.
But the biggest mistake of all was that the British Parliament, in the 1700s, failed to remove the Colonies from private hands and convert them into Parliamentary Boroughs. With that done, a Viceroy could have been appointed to summon a Colonial Parliament with local tax-raising powers and the entire mess could have been avoided. Or more likely, a completely different mess created!
Intelligence, as a survival characteristic, is still very new in evolutionary terms. Much more recent than the ability to run fast, or having big teeth and claws. That may explain our penchant for not getting thngs quite right!