Different reasons for different people. Sometimes people don't know their own reasons but will recite the reasons they like the sound of too. I'm sure altruism does actually factor in, a sense of civic duty, maybe a sort of protective drive, and so on for some police. It's actually a really poorly paying job for how much it sucks - depending on the particulars of the officer's duties, obviously some have it cushier than others. And I'm pretty sure many police officers do not get much excitement out of it - there's a lot of bureaucratic/clerical stuff for many positions, and even police who are "on the streets" more often are likely not regularly dealing with violent crime unless they work in certain areas. I think there are better choices of occupation for action and excitement.
I also think that having a position of authority and status is part of the appeal for some police, I wonder why you don't buy that as a plausible reason someone might become a policeman. It's pretty clear that police can exercise power over citizens.