If breast cancer isn't rare in women under 40, why are there no screening methods offered? shouldn't regular ultrasound or mri be encouraged?

Breast screening: Women at higher risk can begin screening before age 40. This could entail mammography, sonography, or MRI, depending on the circumstances. Although younger women, and men, can get breast cancer, the likelihood is not high enough to warrant a general screening program. The costs($$, unnecessary biopsies, etc...) would not be outweighed by the benefits.