That could be because in many situations (crimes of opportunity) thieves don’t have the luxury of time to evaluate the model of a phone before they steal it. Google needs to step their shit up.
That isn't really consistent with the statistics either: If that was happening then the theft rate should approximate the penetration, but it's still higher for iPhones, implying that the thieves actually prefer them.
That makes sense if they e.g. have a higher resale value, but only if they have a higher resale value and the thieves are choosing them on purpose as a result.