Noob Backup Confusion

Learning about self custody. One thing I'm just not getting. All I need is the 12/24 word seed phrase to get into somebody's account? There's 2048 possible words in each position, so there's a boatload of possibilities, 5.4E+39 if my calculator is right, but is it not possible to choose combinations from random and accidentally discover/access someone else's wallet? There is a boatload of wallets/addresss out there too. None of the pages I've read really cover this.