This is it. The answer. Social Democratic gains in the last century cannot be replicated in this one because we're missing both the Soviet Union and the feeling among the bourgeoisie of that era. They were living in a time where many of them had witnessed with terror the birth of the Soviet Union, the explosion of trade union movements throughout the industrialized world, demands for safety and conditions that were swiftly met. The capitalists were on guard, on the back foot, this communism thing was new and scary, they hadn't figured out how to deal with it yet so they made deals with the workers, appeasement. Also the world after the second world war was for the US and Europe a lot different. A lot of value had been destroyed in the war, there was massive room for growth and high profits, so it mattered less. They've since squeezed out all that growth and are now desperately cutting at things to get more growth because you have to have growth for capitalism. Unless an alien salesperson comes along, sells the capitalists a wormhole to a new world of resources and intelligent beings that can be easily subjugated by our militaries to give them another frontier for expansion and money to bribe the folks at home you're not going to see a return to that. Maybe if there was some massive die-off of humanity as a result of a very deadly plague or nuclear or conventional war that destroyed enough capital it might be able to restart the cycle a bit but none of these things are super likely.

Even if we did experience similar conditions for growth I doubt they'd be as willing to share. They have better tools, a century of experience and refinement for controlling the proles and are cocky because they consider themselves to have won the war against communism with the collapse of the USSR. To say nothing of the mass rejection of Marxism in the west thanks to propaganda which further puts them at ease. They can ignore social democrats/dem-socs all day in the imperial core.