Actually you don’t need the extension for using containers in Firefox, they are built-in.
However the extension is required if you need advanced functionalities like always open a website in a specific container.
I can’t be sure this is the primary reason, but my guess is that using containers can create bugs for websites with subdomains or websites that require you to log in through another domain. For example, if you create a container for YouTube.com and you intend to use it with an account, it will also have to allow Google.com, because that’s how you log in to your YouTube account. The average person likely doesn’t understand the issue or how to get around it.
I currently don’t have Brave Containers, but I am going to guess that it is opt-in and not enabled by default, as the same issues I mentioned above can occur.
I hope Brave improves on the shortcomings Firefox has yet to address with their containers, such as more colors and icons for containers and the ability to add domains and subdomains to a container manually without having to visit that website first, i.e., without needing the internet. This is a problem because some domains immediately redirect to another domain when you visit them, which makes them hard to add to a container.
You can just right click a link and choose open in a container or click and hold the + new tab.
Also right click any open tab and choose to reopen it in a container.