It isn't really that simple. There are other users like satellite equipment & military radar. The military radar in particular runs anywhere they want at anytime without disclosing their actual location.
The equipment itself also has false positives, which lead to much of it being unusable.
This has never noticeably happened in my experience (UK). I've used DFS bands on my routers for years. It's far worse to use non-DFS bands because if you live in a built-up area they are almost always congested, causing poor performance (latency spikes, packet loss, etc).
The equipment itself also has false positives, which lead to much of it being unusable.