There are two ways to interpret and to answer this question.
First, we can say that this question is about why it is a problem when people plagiarize. There are at least two reasons for this. One reason is that plagiarism deprives a student of the opportunity to learn. The whole point of school is to learn things. The things we learn may not seem to be important, but the learning process is important. When we plagiarize, we take away our chance to learn. Another reason why this is a problem is that it is unethical. When we plagiarize, we are taking someone else’s ideas and claiming that they are ours. This is a lie and is not ethical.
Second, we can say that this is a question about why plagiarism is prevalent today. In other words, it is asking “why is plagiarism such a big problem today?” In that case, the most likely answer is the internet and computers. In the past, it was not as easy to plagiarize as it is now. Today, you can just search for your topic and copy and paste other people’s ideas. This is much easier than going to the library and finding books from which to plagiarize.