This is not accurate. While some shared hosts offer node or rails you will be limited by memory and it might not even run your app. The reason PHP is so good for shared hosting is that it runs per request and when you have rarely visited PHP application it requires basically no resources. You can have 100s of small websites sharing resources that get few visitors a day on single server and it will be fine. Try doing that with node or rails which are application servers that need to run all the time.

Heroku is fine but imagine you are agency managing 50 small sites. Heroku is like 7usd month minimum? You can pay 350usd month for Heroku or put it all on 20usd VPS and result will be same.

That's why all self-hosted CMSes worth something are in PHP world and there are very few in python/rails/node.

Also its not like PHP devs have been living under a rock for 20 years and don't want nice things. Git deployment is offered even by many shared hostings, people use SFTP+rsync, or things like https://deployer.org/. And there is very active market of server management tools like https://forge.laravel.com/ https://ploi.io/ https://moss.sh/ that combined with attributes of PHP will give you your own little super easy heroku.

And scaling... well most people don't need scale, you scale CMSes by caching anyway. And you can run pretty big scale just on one machine. And if you want automatic extreme scaling it's not like you can't do that with PHP https://vapor.laravel.com/