Medicine, like engineering, is a domain of trade-offs.
You could order MRIs for every back pain complaint, which would improve outcomes for some % of patients, while probably worsening outcomes for others (due to red herring findings that lead to unnecessary treatments), but at what cost? Who will bear this cost? Regardless of health care system, someone will have to. Most back pain self-resolves with home care, so it makes sense to try that first, unless you have reason to suspect severe trauma that needs immediate treatment (e.g. the patient just fell off a ladder).
You could order MRIs for every back pain complaint, which would improve outcomes for some % of patients, while probably worsening outcomes for others (due to red herring findings that lead to unnecessary treatments), but at what cost? Who will bear this cost? Regardless of health care system, someone will have to. Most back pain self-resolves with home care, so it makes sense to try that first, unless you have reason to suspect severe trauma that needs immediate treatment (e.g. the patient just fell off a ladder).