Photoshop, along with other Adobe software, now uses a subscription based model which users may not be able to justify, unless they're a student or professional constantly using it and recouping the cost.
Adobe also links all of your software and access to your data with a single account password which the company has proven incapable of managing securely.
So those are two good reasons to use Gimp - no recurring fees and no worries about the cloud. Although the best reason not to use Gimp, albeit an unfair one from Gimp's point of view (but still relevant,) is the degree of lock-in to proprietary Adobe formats and tools that pervades the graphics, editing and design professions.
>I have a hard time supporting an Open Source tool that never seems to meet my needs. What am I missing?
Simply having a decent, free alternative to Photoshop should be worth something.