We have our own custom release management software, which now doesn't work. Different repos have to go out at the same time so things don't break. Plus, we extensively use their review apps for code reviews, which we've lost access to.
Lastly, not everyone has access to deploy directly to Heroku, so not everyone would be able to 'git push heroku main'.
Could we fix all of this and get it working? Yeah. But we want to be focusing on building our product, which is why we pay Heroku a ton of money so we don't have to worry about this.