Re: even if the chance of cryonics working is only 1%
They are considered legally dead because we know we have tried reviving people from similar states and always failed.
Doctors are ruthlessly pragmatic (with good reason). They take a great interest in what's going on internally but they don't care about it for its own sake at all, they are only interested in where it is relevant to the questions of "How do I diagnose this?" or "How do I treat this?".
Death means the patient is unconscious and there are no treatment options which have a chance of making him concious. That's it.
It is possible to do scans on the brain to look for activity directly but that would be very very rare. 99% of the time people are declared dead because we know there's no chance of revival. This is a super important point for cryonics because if death is just an arbitrary point along the "a bit under the weather" spectrum, it shifts every time there is a medical advance. That means that the people we freeze today who are 'dead', a 2050 doctor might say had never died at all.