Yes, there is a reason to lock someone out of an account until they can prove their identity. It absolutely does work that way. If security measures are too hard for you to understand, perhaps you should not be using modern technology. If your bank locked you out of your account, you would undergo a series of challenge questions to regain access. If you fail those challenges, no access.
Unless you only had the number for five minutes, you should remember the number. I am 7 decades old and I remember every single phone number I ever had (or I have a record of it somewhere, as they are often used for identity checks). I have only ever had one cell phone number. If someone changed your password without you knowing about it, then that account is compromised, and you should create a new account. It is also quite possible that if someone did compromise your account, they also changed the phone number associated with it, and it would not be yours, so you wouldn't be able to prove it's your account anyway. I don't know why you "need to have all your accounts under one email", but obviously that is not going to be possible. Call Samsung again, and ask for someone in a supervisory capacity or the IT department. Once again, IF YOUR ACCOUNT PASSWORD WAS CHANGED WITHOUT YOUR KNOWLEDGE, IT'S A COMPROMISED ACCOUNT AND SHOULD NOT BE USED. However, I do think that Samsung does have a way to recover the account other than trying to have you remember an old phone number. You never answered the question on why you never changed the phone number on the account.
