Quote from: Coin_Maven on March 10, 2018, 02:50:03 AM

So the software knows a certain combination of 24 words matches a certain keypair.
Obviously the software doesnt have all the possible combinations stored. It generates the 24 words and keypair using certain algorithm.

No.
The software chooses 24 words (out of a set of 2048 words as specified in BIP39 [1]) randomly(!).
Thats 2.96427748 * 10⁷⁹ different seeds. Way more than someone could ever create (by far).

Quote from: Coin_Maven on March 10, 2018, 02:50:03 AM

Why cant this process be reversed?

The software chooses 24 words. This seed is used to derive the private key (or multiple private keys..).
To be more accurate:

Quote

To create a binary seed from the mnemonic, we use the PBKDF2 function with a mnemonic sentence (in UTF-8 NFKD) used as the password and the string "mnemonic" + passphrase (again in UTF-8 NFKD) used as the salt. The iteration count is set to 2048 and HMAC-SHA512 is used as the pseudo-random function. The length of the derived key is 512 bits (= 64 bytes). [1]

Together with BIP44 [2] this basically creates a one-way-function. This makes it impossible to get the mnemonic seed out of a private key.

The public key is derived from the private key using ECDSA [3].
And the address is a RIPEMD160 hash of a SHA256 hash of the public key (hash functions are one-way-functions) [4]

[1] https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki
[2] https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki
[3] https://de.wikipedia.org/wiki/Elliptic_Curve_DSA
[4] https://en.bitcoin.it/wiki/Technical_background_of_version_1_Bitcoin_addresses