Why does "from" email show a different list on single email versus bulk?
Sending an email using CiviMail is different from sending a one-off email to an individual contact, both technically and conceptually. Sending CiviMail is a much more fraught activity in terms of ip reputation, bounce handling, etc. since it is used to send thousand of emails or more. So it makes sense to have different rules, and one of those is to limit the different From: email addresses that allowed.

It sounds like you want everyone in your database to be able to send civimails from themselves? That doesn't sound like what CiviMail is intended for.

Perhaps you're looking for more of a mailing list functionality as provided by list serve type software (mailman, etc.)?