why are women interested in what other people do
People are interested in what other people do. If you're not, you're an outlier. We evolved to our current state in small, hunter-gatherer tribes, and "what other people do" was a matter of life-and-death for us. Study after study has proven that we're intensely social animals. Men are no exception. They tend to gossip just as much as women do. In some cultures, men gossip in different ways from women—and they may not use magazines as a tool for this purpose—but they still gossip.
Unfortunately, almost all my coworkers are men. I've worked in a male-dominated industry for my entire career. I'm around men all day—and have been at five different companies—and they take a constant, gossipy interest in what other people do. Both in what coworkers are doing and what celebrities and politicians are doing.
Both men and women tend to be obsessed in fashions and trends. In my culture, women are generally more into clothes and cosmetics then men are, but plenty of men are interested in other sorts of fashions, including ones centred around cars and electronics. Both men and women spend huge amounts of their disposable incomes on "frivolous" things they don't need, and most are intensely interested in stuff that has nothing to do with survival or improving the world, such as movies, TV shows, entertainment sites, video games, sports, and so on.
And every man and woman I've ever met has been intensely interested in his or her own opinions, so I'm not sure what you're talking about when it comes to that. Most of us care about our opinions and other people's opinions. This is, again, because we evolved as pack animals. It's our nature.
Why do women read stupid magazines?
You mean why do they read magazines that you find stupid? Because they have different interests from you. And, to them, their interests aren't stupid.
By the way, there are lots of women who have no interest in fashion magazines.