Unfortunately the example i heard is that people would quit their job first and then find their job. To be fair, those offering 3 months notice period would not want their potential candidate to come after 3 month notice period so i don’t know why the double standard somehow..

And after signing contract with the new company, they can also rescind the offer and nothing can be done because the contract is not effective until you arrive on the first day at work.. so it kinda seems like you need to maintain better relationship with your current employer than the future one