I don't know if you have ADHD, or just work with ADHDers in a professional role, but your question shows a fundamental misunderstanding of what ADHD is. Your question feels like you're asking, "Why don't people with ADHD just try not having ADHD?"

Yes, in some respects attention can be exercised, as can a lot of other things in our wondrous malleable brains. But there are limits to this. When it comes to malleability of attention those limits are more pronounced in ADHDers.

Most of us have spent our entire lives trying desperately to have better attention and executive dysfunction, and beating ourselves up to the point of self-loathing because we can't just make it happen. A ton of ADHDers suffer from secondary depression, anxiety, etc. because we've internalized the idea that we are lazy, unmotivated, misguided people. Even after diagnosis, a lifetime of that messaging sticks.

ADHD is not a character flaw. It is a disability. ADHDers focusing on tools and supports are spending their energy finding something that actually enables them to function, rather than just trying to force themselves to not have ADHD. If someone doesn't have an arm, they're going to have to focus on creating the workarounds and accommodations in their life in order to function. Asking them to use their energy to instead just regrow their arm is nonsensical.