The thing about senatorial staffs is that most any of those jobs needs a well educated person, and a good number of them would have moved from their home states to DC. So median incomes of an entire city like DC doesn’t mean much.
The $50,000 or so is an average for a staff, meaning many are working for less than that. This average isn’t much more than new college grads average around the country, and a staff would need mostly experienced well educated personnel. It just looks like these senators are trying for a larger staff, rather than a smaller, better paid staff.
Or maybe they all plan to go to work for lobyying firms, or foreign governments, after few years experience, so they don’t mind the modest pay for a time.
A lot of those positions are also clerical and are probably well under the $50K.
When I was in DC in the mid 80’s (I didn’t work on Capital Hill), starting salary was around $14K for a Congressional staffer.