With all due respect 'Gimp works well enough for my purposes' is the issue. Gimp does not work well enough for most professional image editing purposes, audacity doesn't work well enough for any professional audio purposes. But there's a kind of bizarre refusal to improve these programmes because they meet the extremely limited (and often baroque) use cases of their core developers and existing users. This seems to be a major issue with OSS overall. Local maxima. Added to which is the difficulty of attracting UX talent to an open source project, when there's no clear avenue of career reward for the design aspects of an OSS project (vs say the hiring and social honour kudos of being part of an OSS project for a dev).

In terms of commercial alternatives available on Linux, your options are limited for the same reason that so many professional level graphics programmes are not available on Android. Linux users spend enormously less per desk on software, the balkanisation of the platform makes support difficult or impossible. Worse, unlike Android in the mobile space, Linux has so small a percentage of the desktop market that it likely isn't worth developing for (for commercial creative applications).

If you think this situation is the fault of commercial app developers, you're exhibiting exactly the OSS snob attitude I was criticising.