Depending on your environment (outdoors/bright sunlight and angle to the wearer) it can be pretty difficult to notice the light. And not everyone knows what the light is if they even notice it.
That functionality that is supposed to stop the recording if the light is covered is not perfect. You can find lots of "hacks" being offered online and apparently some of them work.
It's pretty obvious Meta did the bare minimum here. It could have made these much more conspicuous or decided, wisely, not to unleash this crap on society given the privacy and abuse concerns that clearly outweigh the benefits.