I fully agree with you. Though Sandstorm in particular explained why they do it this way:

Q: Installing software not managed by the distro’s package manager can be inconvenient for system administrators, especially those who are deeply familiar with the inner workings of their distro.

A: On these concerns: We hear you.

For now, we believe that having our own install scripts allows us to iterate faster, compared to maintaining (and testing) half a dozen package formats for different distros. However, once Sandstorm reaches a stable release, we fully intend to offer more traditional packaging choices as well. We still have lots of work to do!

So, there will be packages once it's stable. Right now, they'd rather spend their time adding new features.