Imagine if you could see atoms that are in the entire universe. Now imagine if you randomly selected one atom out of the whole set. The chances of someone else also selecting the same atom is the same chance of someone else finding your seed phrase if chosen randomly.
This image illustrates this.
source
The chances of a "hacker" to generate a seed that was used before is virtually zero. The only risk would be if you generate the seed by yourself or bad software, seeds with poor randomness. For example, a seed with 12 words like this:
Code:
word word word word word word word word word word word word
Some of those repeated word lists passes the checksum. This is why you should always use a decent software to generate your wallet.