Because a senator should know how to listen to others. They cannot be an expert on everything. Heck, these folks are making rules about medicine, pollution, car safety, tech issues, teaching, school lunches, interstate highways, commerce, military, child toy standards, what sorts of meat to allow in, and lots of other things I can't think up off of the top of my head. No one can be an expert on all of them - why we would suddenly expect them all to be an expert on this is beyond me. IIRC, an education in law is pretty common, which would be expected.

(I know the system isn't perfect and not everyone listens to educated voices and in a perfect world, there would be a range of knowledge in the elected body itself).

Edited for a missed comma.

You need to be a certain type of animal to be a successful politician. Especially in the US where the power per person in the senate especially is enormous.

The house of commons has 600 more MPs for 5x less population.

Because being smart is not a golden ticket or the only quality required to become a senator/politician.

The staffer is a specialist. Intimate knowledge of highway infrastructure, or technology issues alone do not make a senator. A senator is someone who manages a small cabal of specialist experts who collectively hold more knowledge than any one head could ever fit. The job can’t be done by one person, its a coordinated team effort.

would people actually vote for them?