Why do I get data breach notice on all passwords stored on my I phone, but not notices from the source. ?

Because most companies try to hide the fact that their systems have been hacked. See→Passwords & Privacy – Apple Support (AU)

There are published lists of passwords that have been found in data dumps that are for sale on the dark web. Apple compares your passwords against these lists, and tells you which ones are for sale to criminals. You can also check yourself at https://haveibeenpwned.com