If a paper is published in a very good journal (one that is sufficiently good that it doesn't need to accept boring papers to fill issues), that means somebody (reviewer, editor) thought its contributions would be interesting, novel, and/or useful to a nontrivial subset of the journal's audience.
But, this doesn't mean that the paper is interesting to everyone. (Very few papers are!)
For example, I usually find papers that say "We applied a known technique to optimize problem X for some metric Y" very boring, but others in my field appreciate these contributions.
In some cases, you may find the paper uninteresting because you either understood less than the reviewer or more than the reviewer.
- It often takes some knowledge of the field to really appreciate the contribution a paper is making. In most journals, the authors are allowed to assume that the reader already has some field-specific knowledge. If you read the paper without understanding this context, it will probably seem boring or not useful to you.
- It works the other way as well. Sometimes, a reviewer reads a paper and thinks, "This is an important contribution," but those with more knowledge understand that it isn't. For really good journals, the editor can usually find a reviewer who understands the field well, so this (ideally) doesn't happen as often.
I am sure somebody can think of a paper that was accepted only for e.g., political reasons, and is genuinely, objectively unimportant. These are rare enough that I don't think these are the papers you are asking about.