There is no "why". "是" in Chinese does not equal "be" in English. This is just how languages work.
In Chinese, almost anything can follow "是". The part after "是" is just a description or explanation of the part before "是", not an equivalent.
You will encounter many more like this in your study of Chinese. Good luck!
PS: If you happen to know Japanese, thinking about how the part before and after "は" are not equivalent may help you understand (though of course the two languages are very different).
Edit: Since this simple answer gets quite a few upvotes, I think I could add a little more content.
I find the book 实用现代汉语语法 by 刘月华 et al. a useful book about modern Chinese grammar. Though the book is entirely in Chinese, it is aimed at learners of Chinese as a second language and covers many things that learners often find difficult.
The book devotes a whole section to sentences with "是", from page 675 to 689. More than a dozen usages of "是" are listed. Some more examples:
The book argues that these sentences may seem "illogical" superficially and you can often add some words to the sentence to make it seem more logical (eg. "老王是个慢性子（的人）" ), but the changed sentence is often more cumbersome and less succinct. This kind of sentences with "是" (是字句) express ideas more vividly and vigorously while using less words, and Chinese love that.