Disregarding how eerily similar the characters, settings and plots are, Earth having been destroyed and humans being scattered across the galaxy would totally explain why Jim and his mom are the only humans we see in Treasure Planet at all. And not on Earth.
Consider the above... and think about it. It just fits too well.
Humans invented a super-weapon that converts energy into planets. The Drej are a race made up of pure energy. The Drej, upon discovering the fact that a very hostile race had just developed a weapon capable of annihilating their entire race and make Earth-like planets in its wake, preemptively destroyed Earth. Unfortunately, the weapon-ship, the Titan, escapes. Fifteen years later, the son of the man in charge of the Titan project has to set out and complete what his father started: annihilating the Drej and making planets out of them.
- The Drej as victims would make more sense if they focused their violence exclusively towards humans and their allies. When they splattered the harmless The Scrappy all over the wall, they kicked the dog and crossed the line from Good or Neutral into Evil.
- The novelization covers the Drej point of view (seriously, the plot is almost a third longer for it), and this isn't far from how they see things. They don't know the Titan makes planets out of energy, but they know the technology is an exact match for their own technology, and it scares them so shitless that they decide it's all gotta go. As noted above, the queen is willing to risk letting the entire mothership break down and doom them all rather than try to appropriate it to fix the problems.
- Fix what problems?
- A large part of the Drej subplot in the novel is that their own technology has been breaking down for some time and they lack the ability to fix it. By the time the plot happens, the mothership is running on whatever the energy equivalent to duct tape would be; by the climax its so bad that an improvised hackjob is needed to bring the planetkiller weapon back online for the impending confrontation with the Titan. The queen is torn between trying to salvage the Titan and using humanity's version of their matter <—> energy technology to fix their own problems, or destroying it anyway just to be absolutely sure it will never be used against them while praying that they'll find some way to repair the wear and tear themselves before the mothership can't sustain them anymore. The queen ultimately ops for the latter.
- Fix what problems?
- In a nutshell: As noted under "Energy Beings," the novel seemed to indicate that the Drej had mastered the transfer of matter to energy. The Titan Project violated what they regarded as their exclusive monopoly on this ability, which brought their wrath down on Earth and Humanity in general.
- A military superpower fears an economic superpower. The Drej may have feared that humanity's cornering the galactic real estate market threatened their hegemony.