Why don't doctors intervene when there is a 70% stenosis?

Not always curative: In patients with renal artery stenosis several factors are considered to determine if intervention is indicated. If the kidney is already scarred and small and BP is well controlled with 2-3 medications than angioplasty is not indicated even is stenosis is high grade ie 70 % or above.

Renal artery?: Most studies haven't shown much of a benefit to intervention if the other kidney artery is <70%. Sometimes it can reduce number of BP meds and it might be useful if there is only one kidney or if sudden pulmonary edema (fluid in lungs) is a recurring problem. The procedure is risky and often times the risks outweigh the benefits. Hope this helps!

Here are some...: Any finding from imaging studies such as ultrasound, ct, or mri, etc., needs to correlate with patient clinical functionality; in other words, doc needs to judge what it means to the patient and how it can be corrected, not cured. And expected. So, intervention may not be a good choice now. So, it's said: no magic in medicine; if any, that's to apply available knowledge, still... Common sense ; ..

Stenosis of what?: Stenosis or narrowing can be of any tubular structure in the body or blood vessel. Ru referring to a specific artery? Ie femoral artery to leg, aorta, blood vessel in heart or renal artery etc?. Or to narrowing in urinary tract, urethra, or ureter. Or stenosis in GI tract like esophagus or external ear canal? Reason for intervention or non-intervention depends upon structure involved.