Generally they don't unless their doing something illegal. Most politicians are reasonably to very wealthy going into office, and being a high ranking politician basically secures them a high paying position in the private sector afterwards. Many people from the FCC/FDA will go into the telecom/pharmaceutical industries for example. Others will charge huge fees for speaking engagements and appearances. I remember reading Bill Clinton charges in the 10k-50k range for speaches but not sure on the specifics.

So basically no one expects to make money from their paycheck, but rather go into politics for the fame, connections, and other intangibles.

Politicians and their aides consistently get above-average returns on their stock portfolios.

Insider trading isn't illegal for politicians.

They can trade stock knowing certain laws will be passed or bills introduced that will have an effect.

Not being snide, it's a well-known phenomenon.

Look at Harry Reid as an easy example.