I work with US healthcare postings and our numbers are pretty similar: >400k open/active postings, 30% over 90 days, median 44 days. These are direct-hire only postings from the employers (no job boards / aggregators / staffing agencies).

I don't have a hypothesis for why jobs stay posted for so long, other than that employers are always look for a workers, the roles are generally well-defined, and turnover can be high. I don't think there's anything nefarious within healthcare causing so many postings to stay up for so long. Most healthcare employers are generally desperate for workers. So much so that they pay crazy rates for temporary workers (staffing agencies / travelers). The company I work for helps health systems try to minimize those costs.