I think the issue is more complicated than the simple yes/no that a lot of people seem to want it to be.
You give two examples that clearly prove people are willing to show up for an individual movie. I absolutely agree with them. However, those two examples also have a lot of obvious advantages that the average DC film hasn't had. Batman and Joker are wildly popular characters. Joker had Joaquin Phoenix giving an oscar worthy performance and the bizarre novelty factor of Martin Scorsese directing a superhero movie. Etc, etc.
And if we look at the history of the MCU, I find it highly dubious to seriously suggest that the interconnectedness didn't clearly boost the results of movies like Dr. Strange, Ant-man, Ant-man and the Wasp, etc. That kind of interconnectedness that makes people excited is not the end all and be all, but it very clearly is a significant advantage for a movie to have.
The issue is that the WB flat-out surrendered that potential advantage by killing the whole 'what will happen next?' factor for a whole series of movies that were already struggling in terms of gaining any kind of clear advantage in the first place. None of them had world class lead actors, top-shelf directors, or perennially proven beloved characters to work with. The best they had to offer from a marketing standpoint, outside of the interconnectedness, was: the Rock being the Rock; Zachary Levi being Zachary Levi (and Helen Mirren very clearly not being Helen Mirren, performance wise); a nostalgia bait return for Michael Keaton (by far the less famous legacy Batman) in a movie not actually about Batman; the promise of paying cinema prices for a movie with a streaming budget; and the hope that Aquaman 2 will somehow be able to be just as popular as the last one, despite big dumb fun franchises rarely actually gaining popularity over time.
These movies were always going to be hard to sell. The interconnectedness was basically their last best, hope at selling them anyway.
And yes, if the stories had been absolutely rock-solid, peak of the genre, they probably could have overcome all that anyway, through wom and legs. (Maybe BB and AQ2 can still, we'll see.) But they weren't that. And they were absolutely embarassing failures, even in comparison to other films that didn't have absolutely rock-solid, peak of the genre stories themselves.