Why do actors fake type when they're typing in a movie?

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I could understand skits on MadTV or SNL where people fake type but in a movie? Is it really that difficult to type when you're acting? Shouldn't we believe that you're also really using the computer or typewriter (depending on when the movie came out) along with whatever emotion you're trying to convey? It's always made me laugh. I just wasn't sure why they fake type. It's not like they're being tested for accuracy or speed. So why not type out a couple of words instead of just stroking random keys every so often? I think people are more aware of the fake typing because of the internet. You type so much at work or when you're browsing the internet that typing is a part of you. It's not a big deal. I'm just wondering if anyone else has noticed this.