Because you're not used to the idea, and/or you're not picking your route well.

I avoid most arterial streets because the 40+ mph traffic and three lanes in each direction take most of the enjoyment out of cycling. In most cases you can find perfectly acceptable streets that are smaller, slower, and have less traffic, and it's a lot more comfortable for a newer cyclist to start taking the lane.

Be assertive, but avoid being a jackass about it. Make it clear that you belong on the road, and that you know you belong on the road, and you'll be surprised at how many drivers will simply accept your presence on the road.