Built In: The Pros and Cons
The reasons why people stick with the keyboards that are on their laptops to begin with are numerous, and not the least of the reasons is that it's cheaper. No need to go out and buy another keyboard, taking precious dollars away from more important things, and no need to worry that you'll buy a keyboard that doesn't work with your computer; whatever comes with a laptop is going to work with a laptop.
Additionally, you save space too. Since you won't need to carry around a whole other piece of hardware, you can casually slide your laptop into your backpack or laptop bag without fear of squashing anything.
Buying a laptop is a pretty remarkable investment. Within one device you're getting a computer, a monitor, a keyboard, a mouse, speakers, and a battery to make it all portable. On top of that, you often get things like a webcam, and pack-in software to sweeten the deal. But all of these things have a downside: If you don't like them, they're stuck to the computer. You can't just swap it out like you could an external piece of hardware. If it breaks it's much more expensive and time-consuming to replace, often requiring professional intervention if you're aren't an experienced computer technician.
Also, the keys on laptops can often feel flimsy and cause your fingers to slip while typing if you aren't an experienced typist.
On the aesthetic side of things, remember that the keyboard is stuck to the monitor, meaning you can't sit back and relax or back away from the monitor while typing if your eyes get tired. Your hands have to go where the computer goes and vice versa.