Or maybe the intergalactic GPS system happens to be a bit complicated near our portion of space and occasionally a tourist takes a left where they should have taken a right, so they kidnap some humans or cows or something to keep their kids busy while they figure out where they went wrong.Here's another possibility. Imagine if you had a Tesla and went on a trip a couple of states over. You'd have to plan your trip around charging the battery along the way.
Now imagine these aliens are coming from 10 lightyears away, and their destination is a place that is yet another 10 lightyears away. For some reason, their ships get the best mileage out of a fuel created from pureed brain matter. So they find our planet, dip down, snatch up an abductee or two to refuel, and then they're on their way.