Actually the story is even more absurd: the "part they managed to finish" was originally built together with a car and long-distance rail tunnel in the late nineties. Then they were forced to finish it and operate it for a few years as a completely senseless 3-station line not connected to the rest of the network, because otherwise Berlin would have had to pay back the federal subsidies they received for building the original tunnel.
And they completely abandoned the extension further west for which the stations Turmstraße and Jungfernheide were prepared for - it’s now replaced by a Tram line that will arrive there no earlier than 2028
Sure should they - but once not once per person thinking about the same thing. I think a huge problem is availability bias - these projects are/were reported on often because of the delays. The projects that work just fine get a whole lot less of attention.