Just curious: Why does photoshop elements default to Lighten Shadows by 35 percent?

Well, good luck to find an answer to that historical question as Adobe practically never gives details on its purpose when introducing new features. Especially for this very old command inherited from the full Photoshop before the Elements team was moved to India.

There has been a number of similar questions in various forums without any clear answer. Perhaps the most common was to be able to set the shadows value to one's custom preference. So we are left to suggest various possible answers and to comment on the reason of introducing such a feature.

The need to solve the very common requirement of dealing with backlighting and dark shadows was commonly dealt at that time with curves adjustments. The shadow/highlight tool added another important effect (never commented in Adobe's docs) to the shadows, a 'clarity' effect such as what you can get by using the unsharp mask with high radius/low intensity.

It's quite possible that the tool was considered good enough to give satisfactory results with the default settings. There was no change for relatively common change requests, such as creating a preference option to set a 'custom' default value or to remember the last use value next time.

I used to use the tool very often, but not so now that I work mainly in the ACR module, not only with raws but also for jpegs. The control on blacks and shadows is much better.