Seattle Forrest

Any mount can drop something in the perfect storm of wrong circumstances. It's better to drop a rugged device that will survive a fall without much damage than to drop a phone, enough these days can easily cost more than a thousand dollars, and possibly has irreplaceable data on it like photos.

The screen is the #1 consumer of phone battery. GPS might be #2 if it's run constantly. A lot of people want their phone available and ready for the things they bought it to do.

Computers meant for cycling do a better job of it, like specialist doctors are better at specific types of medical problems.

You can get around a lot of this by buying a second phone, an expensive mount (that usually comes free with a bike specific computer), a specialized plastic bag to keep it in, apps to handle your needs, a spare battery, etc. By that point though, a basic bike computer isn't so expensive by comparison.