Which version of WP-CLI are you using? What is wp --version returning you?
Seems a little bit that you may be running out of memory as this is quite a large amount of images and there doesn’t seem some proper batch processing implemented in version 1 of WP-CLI. At least, that’s how I’d understand this issue WP-CLI issue Clear WP object cache periodically on media regenerate/import which I found in the WP-CLI v2.0.0 Release Notes.
Is upgrading WP-CLI an option for you? If not you could also install it as a project-local dependency using Composer. Maybe you need to init a bare bone Composer project with composer init in your WordPress folder first.
$ cd /path/to/wordpress
$ composer require wp-cli/wp-cli-bundle:^2
$ vendor/bin/wp media regenerate