For years the R&D went into making more reliable vehicles which peaked around the turn of the century, then they started trying to appease other interests such as safety (for example more airbags) and fuel economy more, and more recently the slow integration of various things towards self driving such as traction control, collision avoidance braking, self parking, lane assist, etc. These aren't all bad things, but there is no free lunch.
All this creates more failure modes, more weight, but we want crumple zones too, and to still increase both fuel economy and performance. Components, even the sheetmetal, gets mass deducted. Systems get changed like electric steering. Transmissions get more gears and different designs. It all adds up to more complexity and more designs without enough time passing to refine them.
The quality control in the '70s was a different issue and could be worked out through tighter control and robots replacing line workers as much as reasonable possible, but it wasn't just the '70s, rather the entire time vehicles were being made since the beginning, that they just became more complex and customers became more picky so the "error" rate went up.
It's only going to get worse if fuel economy standards keep getting higher. It's a pity that those who think the standards are a good idea, won't consider the cost to the environment to make more new vehicles and scrap old ones because they become cost prohibitive to repair. What those standards really are is a fine, an additional cost to automakers that they pass on to consumers, essentially a luxury tax. The hilarious part is that the difference in emissions between current vehicles and the mandates is less than the emissions from the container ships bringing the vehicles (or at least parts), not even counting the manufacturing or disposal.
On the other hand, you have to expect the manufacturers to progress and for things to become more complex. I just don't think the government should be involved. Let the customers decide.