OK, devs have access to Keychain but they don't have access to 3rd party password managers -- those are independent and the user gives the PW managers access to insert the a/c info into the app. But why can't/won't Apple allow the user to give Keychain permission to insert passwords in apps. That is what I don't understand when Apple allows 3rd party managers to work with apps.
I don't think you understand the way that apps are made. Not every text field is equal. Text fields are used for many things besides usernames and passwords. It is not logical to expect Apple to make all text fields in an app accessible to your keychain.
Also, developers do in fact have access to third-party password managers. 1password is a good example. They have a share extension that can be used in Safari to auto-complete a form with login data or contact info. They also make the extension available in third-party apps. They have a github repo with sample code to help developers integrate 1password logins into their apps. They've been doing this since iOS 8. Here's a video so you can see for yourself:
This isn't Apple's fault. They've laid out the foundation but it's up to the developers to implement it, whether they want to use Keychain, 1Password or XYZ Password Manager.