Because we value our languages and cultures, thank you very much?
Especially if you happened to be born a native English speaking, and you propose this from that vantage point, it comes as borderline racist, and reminds me of this except from Bernard Shaw's Julius Caesar:
"Pardon him, Theodotus: he is a barbarian, and thinks that the customs of his tribe and island are the laws of nature."
English the language of the internet? It is perhaps the language of YOUR internet. The Chinese internet is as big -- and there's the Indian internet, the Spanish speaking internet, and tons more besides.
Why not just get rid of other cultures too, and just keep anglo-saxon culture?
Imagine how easier things could be then, not only we'll have one language, but we could all relate to the same stuff...
Oh, and about those English -- it's dominance in the last centuries is just a historical accident, caused by British and then US power. Even the very term used to describe such a phenomenon, "lingua franca" is not English. The main language used to be Greek, then Latin, then French and English, etc. And that's mostly on the Western part of the world (Europe, US etc) -- that westerners tend to think it's the whole world, but it's hardly 10-20% of the global population. The "lingua franca" could be again very different in 1 or 2 centuries...