Why do members of Congress worry about reelection when incumbents are typically successful at winning consecutive terms?

There are a number of logistical and practical reasons for incumbents to keep their eyes on their constituents to maintain their position. However, the true answer to this question isn't political—it's mathematical.
Representatives in "safe" districts have served for stretches of thirty, forty, or even fifty years—meaning they had to win fifteen, twenty, or even twenty-five elections. And probabilities compound over multiple individual elections. 
Consider a congressperson expecting to serve for twenty years. Imagine that the simple fact of incumbency guarantees a 90% chance of victory in the election. And imagine that campaigning and pandering to the electorate raises that chance to 95%.
90% may seem like a lot, and 95% may not seem like that much higher. But the odds of this representative making it to the end of the twenty-year stretch while campaigning is (0.95)^10, or just about 60%. On the other hand, coasting on incumbency and accepting the 90% win rate plunges the twenty-year prospects down to (0.9)^10 = 35%!
So, to summarize: most American politicians are in it for the long haul. Since even small chances of a loss can snowball over the course of tens of elections, congresspersons are justifiably afraid of risks to their incumbency.