I've got three answers to that and I don't have the patience to blend them all together, so I'll just hit them one by one.
First of all, people tend to vote for the incumbent unless something goes horribly wrong - and even then, they'll usually blame that something on things out of the incumbent's control. That's why we have career politicians: name recognition matters far more than it should.
The second reason is tied into the first, and it's something that we all generally agree on: career politicians are a bad thing. It's impossible to stay in Washington for decades and still remain connected to your voters. Forcing new blood onto the campaign trail also forces candidates to pick platforms and policies that distinguish themselves from the rest. They have to find out what voters actually want, and connect with them in a way an incumbent usually doesn't need to. Primaries become a necessity, and if the current office holder is really so indisputably amazing, the replacements are going to run on the same platform he or she did anyway. If the people want it, they'll vote for it.
The third reason is the same reason we have an Electoral College instead of going by popular vote. Sometimes what the majority wants isn't what's best for everyone (the tyranny of the majority). There are checks and balances everywhere to avoid that, and term limits is one of those checks.
In short, it's the frog boiling problem. People would rather stay with what's comfortable and familiar, even if it's no longer what it used to be and only getting worse. "If the people want it, why not?" Because people are idiots. Forcing churn is a step toward mitigating that.