"Because they are plastic and crappy" would be the general answer.
Ok. Fair enough. But are they really plastic and crappy, or are we just told this by retailers (and lens manufacturers) so that we will upgrade to the better ( and by better they mean more expensive) lenses?
Without getting too far ahead of myself - ultimately when choosing the appropriate lens it's going to be about choosing horses for courses. What do I mean by that? Well, if you are a professional photographer who needs their gear to work flawlessly all day every day, then you're going to want to (have to) pay a premium for the solidly constructed, rugged, weather sealed lenses that cost more than your average car!
But what if you're not a professional? Are you really getting 'better' by spending $600+ more?
Better in what sense though? Better build? OK - maybe. But not by much. Yes, the two kit lenses have plastic lens mounts - so what? The hardened plastic used in lens manufacture has proven itself up to the task, and I'm not going to be changing lenses back and forth a hundred times a day (horses for courses remember).
The fact that these lenses are of plastic construction really doesn't bother me as much as it used to - and in reality you have to spend a lot more (L series quality) to get a lens that isn't made mostly of plastic nowadays.
Okay then - maybe the autofocus system isn't much good on these cheap models? Granted, they don't use the ultra fast, ultra quiet USM systems of the more expensive lenses, with no full-time autofocus over-ride and generally inferior manual focus capabilities. They make a little noise when focusing, usually have the front element or manual focusing ring rotate during focusing, and aren't super-fast constant aperture lenses. But are they also really as bad as they are made out to be?
In all but the most demanding of situations (Formula 1 racing maybe), the speed of the auto focus on these 'cheap' lenses is plenty fast enough. And really, the noise the auto focus motor makes is minimal. I guarantee you that I could use the EF-S 18-55mm at a wedding and never get noticed. The shutter noise of the 5D is way louder than the focusing motor on these lenses. And as for full-time manual focus over-ride... well, I'm sure it's useful for somebody, somewhere, but I've never - repeat NEVER - used it on any of the lenses where it's been available. On modern camera bodies, using even the cheapest of modern lenses, autofocus is amazingly accurate. That's been my experience at least.
So it's got to come down to image quality - right? Time and again I hear enthusiast photographers told to 'upgrade' their kit lenses - as if they've been taking crappy photos for the last year and suddenly all that will change now that they have a 'better' lens. Yet at the same time, many will say that with modern computer aided design, manufacturers no longer make 'bad' lenses. And they'd be right.
I tested the EF-S 18-55mm kit lens against an 'L' series 17-40mm f4 professional lens a year or so back (see the post here) and I was surprised at how close it was in terms of image quality! Yes, the 17-40mm was a tad sharper, with a touch more contrast and a little less colour fringing - but not by a lot. Certainly nothing that a little sharpening and a quick levels adjustment in Photoshop couldn't fix. Yet we're made to feel that if we don't have a $1000 lens slung around our neck then all our images are going to suck!
For the first two years as a freelance wedding photographer, I used Nikon's 18-70mm 'kit' lens that came with the 70D as my main wedding lens (together with the 50mm f1.8 for wide-open portraits), and I never once had a client complain that my images weren't sharp, lacked contrast, or had too much CA. I'm sure I could use Canon's 18-55mm lens in the same way and get equally as good a result.
Now having argued all of the above, I would like to finish by saying that if you're going to pour your money into anything in photography that will make a difference to your images, then pour it in to lenses. What! I thought you said it didn't matter what lens I used!?
No - I didn't actually say that. What I'm trying to say is that the kit lens you own probably isn't as bad as you've been lead to believe it is - and that an incremental jump to a $800 mid-range zoom might not really be worth it.
But what will be worth it, for any photographer, is an investment in fast glass. And by 'fast', I mean constant, fixed aperture lenses that go down to f2.8 or lower. These are usually prime lenses (although fast fixed zooms can be had for crazy prices), and these really will open up a whole new world of photography for you. And they are also an investment. Camera bodies come and go - whereas fast glass often only gets better with age - and tends to hold its price well in the used market. It doesn't have to be horrendously expensive either. A fast 50mm f1.8 will only set you back $150.00 - cheaper than I just bought the kit lens for.
So the next time you think about 'upgrading' your kit lens, have a serious think about whether it's actually worth it - or what you might upgrade to. You may find that it's actually cheaper to keep the kit lens, and buy a 50mm f1.4 or 85mm f1.8 instead? in my humble opinion this will serve you better than any mid-range zoom for $1000 bucks ever will.