Please note that extension update is the same as installing an extension. Joomla runs the exact same code either way. Furthermore, do note that extension installation is performed by code in Joomla itself. We have no say on that code – trust me, I'm trying to convince Joomla core committers to implement a one-line fix to a notorious issue that's been the bane of my existence for the past five years.
Having installed and update Akeeba Backup on a number of sites myself (including this site right here) I can tell you that there seems to be no packaging issue with our software. That is to say, you are not receiving this message because we forgot to add a file or referenced a non-existent file in the XML file that tells Joomla which files and folders make up our extension.
This means that the copy file message comes from Joomla itself, typically because of a directory or file ownership/permissions issue. I have written the following troubleshooting instructions which apply to all extension installation / updates in Joomla: https://www.akeebabackup.com/documentation/akeeba-backup-documentation/installation-updates-upgrades.html#installation-troubleshoot
I hope this information helps!
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!