[danwills]

I think that's a great question!

But I also think it's similar in a way to 'why matter and not antimatter' or 'why ~3d space rather than no structure' (much less obvious the second one!).

But I think the real answer here is touched-on by nmstoker in a sibling comment: We're so far down the energy-gradient of the current-default-chirality on earth, that it would take exploration of an insanely-deep valley for all molecules in a cell (there are zillions even in a bacteria!) to flip the the whole thing to the alternate chirality.

It's a bit like the idea that a good chunk of matter (like even a whole amino acid) could all spontaneously-convert to antimatter at once, sure it's possible in theory but the chances are so low we might as well say it's impossible.

[proto-n]

But then this is also an argument against spontaneous emergence of life, right? Sicne mirror-life could just emerge on its own, instead of evolving from regular life.

[blargey]

Even if the timescales and environments conducive to the emergence of basic life exist on Earth today, I would expect any spontaneously-emerged proto-cell to almost immediately get eaten by normal microorganisms without any meaningful chance to proliferate. Reverse-chirality or no, it'd be a glorified bag of nutrients compared to the more evolved flora surrounding it.

[eightysixfour]

Not necessarily. A pre-condition for emergence and evolution towards more complex life forms can be removed from the system by the existing complex life forms.

At the end of the day, all life is competing for energy. Spontaneous self replicators would be uncompetitive with existing biology and would never make it to more complex stages.