We developed creams that stopped sunburn, but were still great at getting you cancer. A lot of the ads from 70s and 80s talked about how they stopped UVB but let the tanning UVA through! So yes, they didn't burn, but the long term cancer rates exploded, along with all the skin aging.

So it's not that they ignored the all knowledge. I blame most of it on the rather detrimental tech advancement. Like cigarettes with filters: now safer (except you can smoke more now, and the cancer risk is still a disaster)