there are many ways to think about it. but here is one
the 12 word seed is each word=11bits (~1.37bytes)
the first 11 words are random and the 12 word is part random part checksum

basically out of 132bits seedword its 128bits of random=16bytes)

now if you were to try to save all possible variations (not saving the checksum extra bits for efficiency)
1,000keys=16kb
1000,000 keys=16mb
1,000,000,000keys=16gb
1,000,000,000,000keys=16tb

so most hard drives are less than 16TB
thus only 1 trillion keys is more then a users computer can handle (most hard drives are only 4TB)

guess how many possible keys there are
85,070,591,730,234,600,000,000,000,000,000,000,000
                                                     tb   gb  mb  kb

now imagine everyone on the planet(8b pop) did have a 16TB hard drive and everone on the planet filled their hard drive with key variations using a very good random generator

do you know how many planets of equal to earth population there are before 2 people would have the same keys logged

trillions of planets
ill make it simple
85,070,591,730,234,600,000,000,000,000,000,000,000
how much 1 person can store
how much all people of planet can store
how many planets of people are needed        
..
that said
if people personally choose their key. EG used phrases of popular words. then the odds are that someone else might have randomly decided their favourite phrase/quote is the same as someone else.. because phrases are trendy and many good phrases stick in peoples heads and so more chance of collisions

so dont use brain wallets asking you to type in a phrase of bunch of words you can easily remember.. rely on good random generators

yes 12 word seeds are only ultimately 128bit security of obscene amount of variations.. but yes obscene amount of variations of 128bit are more then enough

man, didn't expect a reply like this long. thank you all.
You mentioned good randomness is the key. I can't agree more. There are tons of entropy generators out there, eg coin flips, dice rolls, etc.. Are there some more ways of generating good entropies? for instance with the help of an offline machine, a pi zero, or stuff like that.
I do need real randomness. Don't recommend Ian BIP39, please. Don't get me wrong. It is damn good in terms of giving you a full picture of the key derevation. But I think, it is not a good entropy generator.

Again, I can't thank enough all of you here.

Here is a practical guide where you can learn more about this: https://armantheparman.com/dicev2

Basically you can create a private key using a real world object like a dice or the flip of a coin, etc.

Here's a guide on how to create a private key on a pi zero: https://armantheparman.com/how-to-set-up-a-raspberry-pi-zero-air-gapped-with-electrum-desktop-wallet/