I don’t how it got started, long ago, but I think it persists (among men, anyway), for two reasons:
1. Physical objects that get called “she” (like cars or boats) often have a certain beauty in their curves or lines that, while not sexual at all, can still be understood in terms of desire. For instance, think of how desirable a really fast sports car can be, and how its styling can be fetishized as ”sexy”. And owning a nice car or boat is, in many circles, as much as an emblem as having “made it” as the traditional trophy wife.
2. These things often malfunction at inopportune times, require time-consuming and/or costly maintenance, or (if you’ve spent a lot money, say) fail to live up to the hype & expectations. In short, they can be a bit unpredictable and ”cranky”.
So.. you can see how those attributes match the male perception of the female form & psychology, right? Even if only ironically or tongue-in-cheek? :)
Heh, that’s what I like to think. But honestly, I’d say it’s just because of tradition.