This one sentence—written by George Lucas in 1973—sums up the basic conflict of Star Wars:
“A large technological empire going after a small group of freedom fighters.”
To answer a question like this you really need to look at George Lucas’s out-of-universe inspiration for the whole Star Wars saga in the 1970s: The U.S. war in Vietnam. As explained in Chris Taylor’s How Star Wars Conquered the Universe; bold emphasis is mine:
Lucas was determined now to make movies about war in three modes:
past, present and future; absence, reality an allegory. American
Graffiti would take people back to a time before Vietnam ripped
America apart. Apocalypse Now, which Lucas hoped to direct before or
after The Star Wars, would show it in the present tense. If THX
was the movie he expected to get him banned from Hollywood for life,
Apocalypse Now was the movie he felt wild lead to the government running him out of the country.
The third mode that Lcase intended to use to depict Vietnam—the
allegorical futuristic lens—was only just taking shape, but already it
was being influenced by Lucas’s thinking about the present tense.
Lucas was fascinated by the notion of how a tiny nation could overcome the largest military power on Earth, and this was baked into
The Star Wars right from its earliest notes in 1973: “A large technological empire going after a small group of freedom fighters.”
So knowing that, a lot of the obsession one see’s with villains in the Star Wars universe is based on the arrogance of technical superiority and the obsession of control ultimately being the downfall of the villain more so than the good guys fighting back stronger.
To look at the Vietnam example, the U.S. was trying to stop the philosophy of communism spreading. With the mindset that if China’s Communist philosophy spread across Southeast Asia somehow Democracy would collapse around the world. In my opinion, it’s an utterly paranoid nonsense idea. That somehow if you attack a small group of people who are philosophically opposed to you that you somehow will stop some greater “bad” from happening.
In the case of Star Wars the bad guys are just seen as paranoid control freaks. You’re right, the Rebels are opposed to the Empire so who should care in the end… But that assumes the bad guys in Star Wars are acting rationally when they are not and have never. The irrationality of the Empire—and now the First Order—in their attempts to control what is not controllable is what destroys them in the end.
Also, if you are confused as to why the Empire was obsessed with hunting down the Rebels, think about artists and poets in our real world who simply create images and write words to only be censored, banned or even killed by fascist regimes.
What exactly is so terrifying to a military power about one artist speaking their voice? Easy: The spread of ideas that spread the concept of individuality and personal self-expression terrify people who demand control. Why did the Nazis plunder artworks in World War II and destroy—and sometimes the artists—them as well? Why did the Khmer Rouge slaughter musicians and artists in Cambodia? Same mentality.
Control freaks hate freedom and individual thought.
That said, this basic concept that spurred George Lucas’s creation of Star Wars has been watered down to death over the years. But sometimes the overarching message that folks as plain and simple as a farm boy (Luke Skywalker), a smuggler (Han Solo) and his Wookiee, a gambler (Lando Calrissian) and the Ewoks can somehow aid in the defeat of a massive technologically superior foe like the Empire still comes through.