Part of the reason is that PMR and GCA have a long history of being treated with corticosteroids (prednisone in the USA). The rapid response to prednisone was like a miracle ... never mind the side effects of long term prednisone use.
https://pubmed.ncbi.nlm.nih.gov/16901030/
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PMR was never given much thought by the medical research community. However, doctors in general dislike prednisone as a long term treatment for any disorder. Unfortunately, prednisone was called "the only treatment" for PMR because, until recently, nothing else worked well.

That all changed in 2017 when Actemra (tocilizumab) was shown to be effective for GCA. The FDA approved Actemra (tocilizumab) for the treatment of giant cell arteritis (GCA) in adults on May 22, 2017 based on the GIACTA study results.
https://www.fda.gov/news-events/press-announcements/fda-approves-first-drug-specifically-treat-giant-cell-arteritis
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Five years later, the SAPHYR Study showed that Kevzara (sarilumab) was effective for treating PMR. The FDA approved Kevzara (sarilumab) for the treatment of polymyalgia rheumatica (PMR) on February 28, 2023.
https://www.the-rheumatologist.org/article/fda-approves-sarilumab-for-the-treatment-of-adults-with-glucocorticoid-resistant-pmr/
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Prednisone is still considered to be the "first line treatment" for PMR and GCA simply because it works quickly to relieve the symptoms. The problem is that patients are unable to taper off prednisone quickly. The side effects of long term prednisone use are notorious in the medical community.

Many doctors and patients are set in their ways. They are wary of medical breakthroughs so doctors leave their patients on prednisone. The other reason is that biologics in general are expensive treatment options.

Probably protocols to try other drugs first!