Because HDR wasn’t natively supported on most displays and software, for a long time it was just “hacked in there” by squashing the larger dynamic range into a smaller one using a mathematical transform, usually a log function. When viewed without the inverse transform this looks horribly grey and unsaturated.

Directors and editors would see this aesthetic day in, day out, with the final color grade applied only after a long review process.

Some of them got used to it and even liking it, and now here we are: horribly washed out movies made to look like that on purpose.