A seed phrase usually consists of 12 words. Considering the 4 bits of checksum, these 12 words generate 128 bits of entropy.
This means that there are around 3 * 1038 valid 12-word seed phrases.
It's extremely easy to enter a valid seed phrase.
It's extremely unlikely to enter a seed phrase already generated by someone else and steal the fund.
In other words, you can enter a valid seed phrase and generate valid addresses. But you can't generate funded addresses.
Using just the valid words what is the likelihood of opening a wallet?
Obviously not hard as I did it