I think you what you mean is "why certain repositories are not available by default?"
The reasons may have to do with control over how the developers can distribute their software, analytics and staying more up to date.
For example, there are multiple release channels available for Brave Browser i.e. Stable, Beta & Nightly. Having a dedicated repository for each of these, allows users to download exactly the version that they want. It's easier for the developers to have a rough idea of how many people are downloading each version, even if there's no other form of telemetry involved.
Another reason this may be interesting to the developers is to keep their packages release cycle independent from the official repositories. What I mean by this is that Ubuntu pins down certain package dependencies to a specific version. This is done to ensure stability of the system, and sometimes it means that certain packages cannot be upgraded beyond version X even when version X+1 has been released. Even when the developers claim that version X+1 works well.
By providing their own repository, they are taking the responsibility of testing the package to ensure that even people using the older versions of (still supported) Ubuntu LTS versions can still benefit from the latest versions of their software.
You could then ask, why don't the developers of distribution XYZ add these repositories themselves? Well, that's up to each one of them to choose. Zorin OS decided to do this and add Mozilla's repository by default given the number of issues reported due to the Flatpak version that was used soon after the release of Zorin OS 17.
No, I meant to ask what I asked. Some .deb files will add their repository. Others HAVE a repository, but it must be added by hand. If the user is going to a) install a deb and b) have a repository added, why do some devs not include adding the repository in the .deb, thus reducing steps and risk of human error in adding the repo?
I'm just guessing here but maybe is to avoid problems if and when the user updates the operating system to a higher LTS version. Repositories aim at specific versions of Ubuntu like focal, jammy, etc. If the user upgrades without also making sure these are upgraded, it can cause problems.
Google Chrome is a good example of this, as it explicitly informs the user at the download prompt that the repository will be added and provides instructions on how to prevent this.