Why do I rarely come across 'using namespace std?'

For context, I'm pretty new to this language. I'm about halfway through 'C++ A Beginners Guide by Herbert Schildt,' but I have explored the language past this book (embedded things).

In the book, the standard namespace is used for every program.
In C++ programs that I typically see, std:: is used (e.g., std::cout).

Is there a disadvantage to using the namespace? Is it that it's outdated?
(The book im reading is from the early 2000's)

https://redd.it/1pubujb
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