They leave outside a lot of information
I was researching for a paid article I am going to write on cognitive bias, when I remembered that I really dislike abstracts for researching my articles.
I get bored the same second I open an abstract. I just see an enumeration of concepts, empty of reasoning and content, and then I scroll, hoping to find something interesting.
I never do.
The reason is that it simply bores me to death, and I end scrolling to the end of the summary without reading a single word.
Those abstracts are empty of all emotion, they do not sound human, and when you want to deliver information to the public, you need to make it in a way that makes your reader resonate with it. It must be easy to read, not a technical manual. Anyone who looks for technical manuals will never look for an abstract. Trust me, I used to read those.
However, the main issue with abstracts is how to define what is filler content. You should be able to reduce everything you write to the minimum, but this does not mean that you should cut a text into little meaningless pieces, not only because you will lose some small nuances after the trimming. But also because we do not see the same parts as important and what might interest who reads the book in the first place might be irrelevant for the person reading the abstract.
Full books deliver you all the information unfiltered; They include every piece of information that the original author wanted to give you, delivered the way the author intended it to be, so the reader understands its meaning; and once you read it, it is up to you to decide what is useful and what is not, while they still allow you to scan them. Nowadays, with digital books, you even can type what you are looking for to jump there. And for the rest of the searches, there is Google.
If you have the time, and you like to read, you will profit more from reading the full book from start to finish to find later your own conclusions on the matter.
To me, there is no simple way to learn things. Summaries are useful to learn a few random facts to spew at parties, but do not deliver beyond that because they do not dive deep into the topic.
It is always better to read the full text on everything, even if some books could be shorter, it is still better to have available every minor detail to understand the topic you are reading about.