I would like to ask why mega journals (large journals designed to be much larger than a traditional journal and with a very broad audience, e.g. Scientific Reports, PlosOne, BMJ Open, Heliyon, ... to name a few) are so popular / receive so many reputations. Why do people choose such journals over field-specific journals. As for me, I would never go to the homepage of Scientific Reports, Heliyon, etc. to screen the most recent articles because the subject span is to broad for me. I understand that their acceptance rates are a crucial factor - but is there anything else I did not consider?
I never go to e.g. Nature to skim through it in hopes to find the paper that matches my field, however, I tend to be aware of such papers. They tend to be quite public at least within the research field communities.
Which leads to what I think its a conflation of concepts you are making in your question. They are not reputable because they are a good source of information for all fields, on the contrary, they are reputable because they only publish "groundbreaking"* research, from any field. Prize-deserving research. So it is reputable to get a paper in Nature because you got in, not because other people from your research field will read it. Its the exclusivity that makes them reputable, the "stamp of approval" that a publication in these journals gives you as a researcher, and to your field, because its considered "important enough" to be on Nature. In some way its a lesser form of a price, as very little people get in.
This does not mean I agree with such "reputation-based" system for academia. But the truth is that academia works a bit like this, and the above paragraph describes how many people see publications in these journals.
*In theory, of course, not saying this is necessarily true.
It is the Matthew effect.
A reason some people select a journal to try to publish in is prestige. A reason some people judge a journal to have prestige is the famous articles published there. This provides a positive feedback. A famous article makes the journal more prestigious. Being more prestigious attracts more attempts to publish by people doing interesting work. If the editorial process is handled with care this can produce a journal that becomes more and more prestigious.