Because you cannot save enough cash to be able to afford every worthwhile treatment through up-front payment. If you don't have $150k in cash and don't have insurance, you can't get chemotherapy and die a preventable death. With insurance, the worst case is paying your out of pocket maximum, getting treatment, and then declaring bankruptcy or otherwise fighting attempts to balance bill you for hundreds of thousands of dollars.
Your assumption of $150k for chemotherapy treatment is exaggerated, especially in light of Trump's executive order to try to get price transparency at the level of actual insurance reimbursement.
$25k/year over ten years is $250,000, and compounded at 6% is $319,000.
So yeah, I'd easily be able to self insure for anything.
Because negotiated prices aren't actually low for a good chunk of treatments - especially for the health conditions that someone paying $20k+/year already in premiums is more likely to contract.
I pay $25k/year for family insurance with high deductible, and no pre-existing conditions.
Negotiated prices paid by insurance actually are pretty good, but could be even better with the price transparency that will come about with Trump's executive order.
Why pay $20k+ per year in premiums when I can just save and invest that and pay low prices when needed?