"Why can't I just solder a microchip, serial port, and battery onto a circuit board?"

Do you know how to solder?
Do you know how to design a circuit board?
Do you know what constitutes a "serial port" in terms of electronic components on a circuit board?
Do you know how to debug hardware if it doesn't work?

While it is, of course, possible for you to do this, don't you think it might be wiser to start with something professionally designed & built - with instructions, support, and a guarantee - so that you at least have some confidence that the hardware actually works?

"Wouldn't this be a lot cheaper?"

Not necessarily; eg, Hitex sell a development board for the Philips 89LPC932 for only GBP 39.95 - about US$ 66.00.
How much is your time worth?