Thy aren't "junk" but they are 1) extraordinarily more complex, and 2) not given as much development time because there's always a new requirement to meet.
Contrast new engines with something like a small block Chrysler. Basic LA engine dates to the 1964 273. Stroke it to 318 in 1967 (the LA 318 was quite different from the A version, despite identical bore and stroke). Bore it out to 340 in 1968. Make a 360 truck version in 1971.
Go to a hydraulic roller cam in 1985. Switch to the "magnum" style heads in 1992.
Yes, there were camshaft changes, some heads had different valve sizes. Different carburetors came and went, it got fuel injection at some point, and so forth.
But it stayed in production until discontinues in 2003. Basically 40 years of development with only minor changes. One major round of emissions changes.
Contrast that with a modern engines, which are maybe only have a useful life of 5-10 years. An OEM has to go super fast to bring the product to market because of the short period of time over which you'll be allowed to sell it until it's regulated out of existence. It's launch-and-move-on. They won't be able to develop that engine to a finely polished gem because that takes time and money. And because it takes time, that means you need stability. Stability is sorely lacking in a regime where nothing is ever clean enough regardless of cost.