with interest being zero as it was the past couple of years, can the bank not just sit on that money and literally do nothing? What operational expenses do they have?
Sure, if depositors said "ok we'll let the bank ride this out." But that's not what happened.
Yeah, and remember that this particular bank has a very, very well-connected set of customers (in the graph theory sense). So once the run starts, everyone tries to get in, causing the actual run.
rent and maintenance if they have branch property, salaries and benefits for staff at least. Banks have a lot of regulation and audit requirements which take work.
but that doesn't increase with more deposits, right?
if profits don't go up, shareholders get mad and the stock price tumbles. this is just capitalism.