It is usually not THEIR money that they are spending. It is contributors' money. Contributors give money to politicians for two reasons: 1) to get something, a law, a project, whatever or, 2) to get the politician to leave them alone.
Then, their salary is only the tip of the iceberg. They get various perquisites such as office expenses, "fact finding" travel trips. The biggie is a pension. And their salaries and pensions are much larger than those an average person or even a mid- to high-level exec in a real company can expect to get.
And, don't forget -- while every penny we peons move in and out of our accounts must be accounted for, the big perk for them is graft - bribes -- for their votes.
Oh, then after they leave office, speaking fees, sinecures in various government do-nothing or busybody agencies, or law firms with connections. And don't forget the money-laundering via book contract advances for their memoirs.
And another, access to star-struck, power-worshipping interns.