But why do they?

Is traditionally answered with "money", but that's not really it just now.

The combination of biotech, ubiquitous computing/vastly better communications technology, and the angry sky ("anthropogenic climate change") are going to compel economic re-organization on the scale of the industrial revolution. (Two generations of wars, bloody revolts, and remarkable nastiness on the part of the entrenched landholding ruling class; another generation or so the entrenched landholders refusing to share political power; a generation after that of great political upheaval. And that's the United Kingdom, 1775 - 1875, more or less the best case until you get to things like Taiwan where it was done with careful purpose in the 1950s and still cannot properly be considered "gentle".)

Lots of people, the great majority of people, who presently belong to the ruling classes (whatever their job title is; the folks who make policy and have it stick) are aware of this, and are going la-la-la with their fingers in their ears about root causes. (No matter what, they're going to lose status, and no one stays in that class without being intensely concerned about status.) They're also wildly unsure about what to do about the root causes; nobody stays in that class through being comfortable with making mistakes, and it's pretty much guaranteed that waiting until there's enough information to be sure what the right policy is would itself be a mistake.

So, instead, by a sort of social reflex, they're cranking up the oppression; secret trials, police check points, "papers, please", ubiquitous surveillance, because, hey, when the revolution comes they want to be ready.

Very predictable; also very stupid.

That economic transformation is also an immense opportunity to make people materially much better off, increase the general standard of living, and to wind up with a whole world that's educated, healthy, and has interesting stuff to do.

We need to insist on a ruling class that's in favour of those things, instead of opposed.

That would require not just a message of hope but a goal of progress, and if the deity of progress died in the mud of Flanders, well, that's a god people made, and what was made once can be made again. It would require a recognition that, yes, democratic constraints and reduced—hopefully greatly reduced—relative economic standing and being constrained by facts are all constraints on power, and very sharp constraints on relative power, and you still wind up with more actual power and resources to direct by any absolute measure than you do by keeping things from changing.