I don't think it's as small as the oxygen molecule example that I gave (though I've never tried to estimate it, so I suppose I could be mistaken about that), but it definitely is plenty small enough to also be considered "not possible" by any reasonable person.
The oxygen example is an extreme one. Because I'm a nerd who loves this kind of stuff - some very rough calculations would put a small 5m*5m*3m room at 75,000 liters, 21% O
2 gives 15,750 liters, with the molar gas volume of 22.4 liters at STP giving 703.125 moles of oxygen, times Avogadro's constant giving 4.234*10
26 molecules of oxygen. If you give each molecule a 12.5% chance of being gathered in a specific corner of the room (given that there are 8 corners), then your chance of them all being gathered in same corner is going to be 0.125^(4.234*10
26). My software won't calculate that number. I get as far as about 10
-1,000,000,000 and then it gives up and says zero. Heh.
So yeah, a bit on the extreme side, but the principle is the same as I outlined above. Even if everyone in the world did literally nothing but constantly generate new wallets for millions of years, we still wouldn't get a collision. It is safe to assume the chance of a random collision is zero, just as it is safe to assume the chance of randomly suffocating is zero.
It's probably worth pointing out that if you think a 12 word seed phrase is insecure, then swapping to 24 words doesn't change anything. Bitcoin private keys "only" provide 128 bits of security at most, regardless of the number of bits in the seed phrase used to generate them. If you think all private keys are insecure, then your best mitigation to this (other than learning the math to see why they are not insecure) would be to use a multi-sig set up.