Let's say we have a trillion planet Earths. On each Earth, there are a trillion people. Each person has a trillion computers. Each computer generates a trillion keys a second. All these computers have been creating a trillion keys per second since the birth of the universe 13.7 billion years ago. 10^12 * 10^12 * 10^12 * 10^12 * 60 * 60 * 24 * 365 * 13.7 * 10^9 = 4.3*10^65. This means thay they would have so far generated approximately 0.0000000004% of all private keys.
Is this really correct?
This is very impressive, and it really shows how our human mind can't understand what big numbers really and how they work.
Now, you don't actually need to "know" your private keys in the sense of having them written out in plain text and saved somewhere. You simply need to be the only person who can access them. Just in the same way, you don't actually need to "know" your seed in the sense of having it memorized, as long as you are the only person who knows how to retrieve it from your back ups.
It is important to note that if are you holding significant amount you should really do that, hardware wallet.
However, if you have a family it is important to allow a familiar member know how to retrieve them as well, especially if you have kids.
Those coins might be worth a lot more in the future