There are a lot of potential reasons.

Here’s a thought: consider the potentially analogous case of performance-enhancing drugs for athletes. The drugs unambiguously make them better at their jobs, but the drugs have severe long-term health costs and wreak havoc on the fairness of the playing field. It’s easy to see why an athlete might choose not to use them, even when others are.

Of course, those negative factors alone are not enough to dissuade people en masse who want to get a leg up on their competitors, so the use of performance-enhancing drugs must be further restricted by institutional bans.

It's not actually useful for me. But I still use it anyway.