Why, you ask? I'll tell you why. First off, they don't have magic. Obviously. That's the biggest reason, really. Muggles can't fix broken things with the wave of a wand. They can't make something happen in an instant, nor can they slow down an action when they want to. The other stuff is just minor stuff. Those vermin rely on giant motorized metal boxes to move from one place to another. They can't fly broomsticks. Muggles hoard broomsticks for nonsensical uses, such as cleaning, something we could accomplish with a swish and flick. They don't even have house-elves to clean up after them. Bah, as if their filth could be cleaned up anyways. Even a Cleansweep 7 can't help their case. 
Interspecies breeding with wizards is just another way for Muggles to somehow heighten their status in the world. This futile attempt at getting on our level is simply pathetic. It only causes a problem on our end, for we will have to weed out the infestation of Muggle-borns that is steadily increasing in our world. Luckily, we have an easy way to identify ourselves amongst our peers. It is called the House of Slytherin. As you all know, Slytherin was the only Pureblooded founder of our great Hogwarts. He embodies what all of us as Purebloods should strive to be. Some aspects of the Slytherin House are noted to the right. These wonderful qualities are strictly Pureblood qualities. You would never see one of these characteristics in a Muggle. Such are qualities that come only from high breeding and growing up in a proper family. 
Muggles come from disgusting beginnings, and therefore, can never rise to the class of Purebloods. 

Dear Muggle-Borns,

You are dirty beings that pollute the magical world, my world. When I say this, I believe I speak for all Purebloods: You don't belong, so please, kindly remove yourselves before I get to you and must forcefully eradicate you from this world. I'm sure you wouldn't want that to happen. To be honest, neither do I, because I don't have the time to waste on all of you. As much fun it would be for me to dispose of you, I have more important matters to attend to. With Harry and his meddling friends on my back, I need to make more Horcruxes, but I can't do that if I'm still on the search for the Elder Wand. See? We're all busy. If you do your part, then we'll all be much happier people. 

I don't know how you managed to steal magic from a witch or wizard, and how their wands works for you, but you, simply put, are an abomination. You give us magical-folk a bad name. Your filthy selves are somehow infiltrating the Pureblood domain. My blood status interrogations will help identify those who belong and those who don't. I'm not sure if you know, but only Purebloods are allowed at Hogwarts now. Sorry (not really). Muggle-borns just don't deserve to know magic like we Purebloods do. You have not done anything to be worthy of our magic, nor of the knowledge that we possess. Your meager minds cannot manage the degree of power that our minds can. 

Yours truly, 
Lord Voldemort. 

P.S. The sooner the better. Again, for both of us.