People do it because every other option sucks.

Sharing to someone else's timeline is way too invasive, sharing to your own timeline and tagging the person is weird, sharing via message is too disruptive.

What people is looking for is a "Show it to a friend" button that just creates a notification on the person's bell icon and allows them to respond in any way they want.

I share via message. It essentially just creates another notification like you suggest.

But would it increase engagement? Number of comments is one way we measure engagement and you’re advocating for a feature that would take away 90% of those comments away? Increase engagement at all costs this is the only way.

Easy and expected solution: they should count the number of Share via Message into the share count (even if it is shared privately).

Are shares not as good of a metric as comments (even though most comments are effectively shares, making them even more noisy)?

Simply put: UX is bad and most people figured out this was an easier way to do it. (Also something that comes to a shock to people working in the Bay: Most people are really not that good with computers).

Also, basic behavior mimicry. You see other people do it so you also do it.