...because we have documents (e.g. the Federalist Papers [1]) that explain their philosophy and arguments. Moreover, significant technological advances aside, our basic psychology / neurobiology remains virtually unchanged, and so many of their initial insights into mitigating the risks of human political systems still pertain.
For instance: they foresaw the problems powerful interests acting in bad faith could cause, and so we now enjoy judicial recourse when politicians or appointees make arbitrary, capricious, or corrupt decisions. The fact that we're discussing legal challenges to the FCC's decision as even a possibility underscores this point.
We understand more about human psychology / neurobiology now, of course, so this is one limitation of uncritically accepting their advice. We also have the benefit of over two centuries of additional hindsight. Still, I think there is good reason to at least consider the opinions of people who would, by any reasonable reckoning, count as political systems design experts of their time.
[1] https://en.wikipedia.org/wiki/The_Federalist_Papers