why does apple forget my password ???

You are sign in here .  .  .  with your Apple ID & Password.

Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.

You can’t merge multiple Apple ID accounts.

What happens when you use the same Apple ID & Password that you use to sign in here?

See the following Apple articles:

Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support

If you can't connect or sign in to iCloud -> If you can't connect or sign in to iCloud - Apple Support

Have you forgotten your password?, See -> Change your Apple ID password -> Change your Apple ID password - Apple Support

Then see this Apple article: What to do after you change your Apple ID or password

What to do after you change your Apple ID or password - Apple Support

I’m having the same problem on my iPhone 7? Have you found a solution?

There’s no reason apple should force clients to change their password daily’ Why?