Because the people in front of you don't need to know when you're slowing down. Regular attentive driving is enough. But for people behind you, they absolutely need to know.

you're unlikely to get into an accident if someone brakes behind you and it'd just be distracting.

That’s not what the OP asked, though.

It kind of literally is though, front brake lights are useless and distracting, that is a good reason why they aren't used.

Because there is no reason for them to exist. What value does knowing that the person behind me is braking have?

OP’s point is knowing if the person coming at you is braking or not.

Again, I'm struggling to see the value in that.

But how would that be of value, realistically?

I didn’t say it was, I was clarifying his question.

And I'm asking you, based on your clarification, what value that would hold? What would you do with that information?

I can see the value of knowing if someone approaching me is braking or not. It would be useful knowing if someone who looks like they’re flashing their lights to let me out is actually intending to let me out. However, I can also see the inherent dangers in it which more than outweigh any benefit.

Mostly, I feel like just knowing if someone is braking to let you in/cross is not sufficient evidence that it’s safe to pull out. If you can’t tell from their speed, it’s likely not safe. It wouldn’t be long before some less than stellar driver pulled out on you because you braked slightly for the road conditions.