Because you can't instantly erase someone from your perception in person to person interaction. Online you can even proactively protect yourself, such as by adding words and phrases you don't like to your blocklist. Suppose I decide I don't ever want to interact with someone who still uses the word "retard". My computer can preemptively block those people on my Twitter, my email, even pages containing the word on my browser. It takes only seconds to wipe entire categories of people and opinion from every facet of your online experience.