Because you probably didn't have one. Ms. Hume was my 10th grade English teacher, and her class was notorious around school for one reason: she forbade any form of the verb "to be" in the papers she assigned.
Is, was, has been, is [insert verb here]ing. Those and their pallid brethren would get you points off. Big points.
Can I just say that this was one of the most important educational experiences of my life? Even now, I feel a twinge of icky guilt whenever I type "is" and "was" (and yes, you'll find those in this post and elsewhere in my work). Those words? Lazy. Weak. Wan. They permit passive, boring sentence construction and dull the energy of the language around them.
Sure, my classmates and I tried to find the easy way out ("seems," "feels," "appears," oh yes, we used 'em all), but Ms. Hume's ultimatum meant that our impressionable little minds had to work to find the best, richest words for our treatises on Wuthering Heights and whatnot.
Ms. Hume's class made us mindful of the words we use and how we use them, and that, I've never forgotten. I owe that woman a lot.
As writers, this mindfulness is our job. Sometimes "is" is the word you want, but decide that consciously. Words, and the choices we make when we fill a page with them, are our only tools. Language holds a lot of wonder for those willing to put out the effort to uncork it.
Try this. In your next piece of writing, whatever it is, don't use any form of "to be." See where that takes you. I bet it'll be (yes, be) someplace good.