Parallel universes were different from the main universe. They were equivalent to fan creations of the main universe.
Just like most fan creations were either poorly made or ended badly.
Parallel universes with complete "settings"—such as stable physical laws, an endless technology tree, and a rich, grand history—were extremely rare. Even if one existed, it would be closely monitored by the Space-Time Administration.
Unlike "multidimensional space-time," the resources, technology, and population of a "parallel universe" could seamlessly replace those of the main universe.
That was why there were so many incidents of "parallel-universe smuggling." Most of the time, advanced civilizations that discovered something was wrong with the place they inhabited tried to advance from "fan creation" to "official canon."
As for ordinary attempts to reach from the main universe into other parallel universes, they were all for conducting "civilization experiments."
The great engineer's book-selling venture in his previous life could also be considered a kind of "civilization experiment"—an experiment in preferences.
But that also meant tremendous risks.
Because not every parallel universe followed the same laws as the main universe.
For example, some parallel universes were naturally "dead universes," with no soil in which life could survive. Let alone ordinary life—even ninth-level lifeforms would die instantly in such a parallel universe.
And these "dead universes" accounted for no small proportion of all "parallel universes."
Apart from "dead universes," there were "life-bearing universes."
The only thing these universes had in common was that they possessed life.
And every one of these universes was a "rich mine." Even from the perspective of an advanced civilization, they were still "rich mines."
How large was a universe? Even if resources were produced in only one ten-millionth of its territory, that would still be enough to make one prosperous.
In his previous life, the great