Old energy accounts closed when switched?

Parwood

Hi everyone, Just joined, first post, and found nothing about this topic when I searched, so here goes!
Wanting to avoid TalkTalk's recent introduction of charges for email addresses, I'm having to change those I use a lot for online accounts. Looking back through accounts with previous energy companies, I tried logging on as before and was surprised how many I could still do so: 4 out of 7. Also surprising is the general absence of a 'close account' option in their 'manage account' sections. Often it seemed the only way to do it while logged on was to say either you had moved house or died!
Of course, I have emailed them directly to ask for the accounts to be closed, but also for any reasons why they should hang on to my details (i.e. keep the account alive) once I'd switched, especially in the light of GDPR legislation which, I believe, indicate that they should have reasonable need to retain my personal details and data. I don't feel this is justified.
Has anyone else had similar experiences, and/or is able to shed light on why companies do this and if they are allowed to?
Many thanks.