Different districts have different physical servers and they do not talk to each other. This is by design for security and not an oversight.
While not exactlly convenient, separate access accounts provide an additional level of securty in regards to students' educational records, which are protected under a Federal law called the Family Educational Rights and Privacy Act (FERPA).
And before you ask, no I do not work for PowerSchool. I just support PowerSchool districts.
Thank you for reaching out in the Community!
Thank you for your help @JeffG9 !
@JeffG9 is correct! If you have children that are no longer in a school district, I suggest reaching out to the previous school district to request your account is removed within that district. This will allow you to use the email address within a new school district.