Why didn't the Founding Fathers foresee/prevent the Swamp I.e. the mega corp lobbyist govt complex system military media financers Fed
They did, in a way. This is partly why they insisted on small, less-powered, federal government.
Because the only practical way to prevent lobbying, is to eliminate the things for the government to be lobbied for. As long as you have a big, powerful, and money-rich government, lobbying is worth the effort and therefore basically inevitable, in some way, shape or form (the return on lobbying investment is always good).
Additionally, they explicitly made numerous references in debates, and provisions in law, to prevent the big perceived threat - lobbying by foreign governments.
As a side note, "mega corp" may sound like a nice populist slogan, but AARP is a major lobbying force that is the cause of one of the biggest (some would argue THE biggest) strategic issues facing US economy - the enormous fraction of US budget that is spent - and even bigger one projected to be spent next 30 years - on social support of retirees. Yet they are not a "mega corp".