However, I do not expect the average congressperson to have the level of expertise in multiple disciplines (like biochemistry, oncology, endocrinology, etc.) required to make decisions about these sorts of things. I'm also not inclined to think that the average congressperson would do the right thing in regards to regulating business when large donations to their campaigns from those industries are involved.

Except nothing in the US is a direct democracy already?

Your argument falls apart because the entire point is that we elect representatives. Those representatives can not do _everything_ themselves.

Even the most basic city government has elected and appointed positions.

Do you elect every single cabinet member into office? How about all the people that work in those offices?

Everything in politics is about delegating authority. The bureaucrats were indirectly elected by the bureaucrats we did elect who gave them their authority. It's also why citizens don't vote on laws themselves.