That’s a ton of manual labor on a disposable container. Manual labor is extremely expensive, and plastics manufacturing is extremely repeatable and extremely cheap at scale (that’s kind of the big reason plastic is so common, it’s cheap and reliable)

Granted I could imagine a machine which fills it with dyed water, uses a camera to determine the fill level then passes to another machine which adjusts by volume but that’s really going to slow down the production line doing all of that.

Ultimately plastics manufacturing is way more precise and consistent than that, you’ll basically end up with either a milk jug or a misshapen mess if there is a defect, and not really anything in between