That just isn't true; you still need a flightworthiness certificate from the FAA in order to register and (legally) fly the plane. The exceptions are for ultralights, sport aircraft, and aircraft which remain tethered to the ground (see: moller Skycar) or never leave ground effect (see: hovercraft and ground effects planes such as the Ekranoplane - which would be registered as boats).
That isn't certification, though. A certified aircraft (anything factory-built, basically) has to meet very particular standards for performance, function, reliability, etc. That takes a lot of paperwork and testing (I know this because I am an engineer at an aircraft manufacturer). It also requires very tight control of the manufacturing process, and requires that any modifications or deviations be approved.
Homebuilt aircraft are given airworthiness certificates, but that is expressly not a certification. There's a little formality to it, but it's basically a quick check to make sure you didn't do something horribly obviously wrong (like forget to hook up your controls, or leave a wing off, or forget your basic instruments). You're also required to perform some level of test-flying. And even after that's done, you can't carry passengers or cargo for hire, or use the airplane for any commercial purpose (like banner towing or aerial photography).
With a certified light airplane, you get a guarantee that the aircraft meets certain performance and safety standards, a proven flight envelope, greater flexibility with use, and a wider resale market. However, you have to pay a licensed mechanic for all your maintenance (including annual inspections) past things like changing your oil, you have to follow the manufacturer's maintenance program, and making changes or modifications (or even buying replacement parts) can be very expensive.
A homebuilt airplane will generally give you better performance for your money in terms of range, speed, payload, maneuverability, etc., and there are many more options to choose from than on the certified market. It is also much more customizable (you can fit any equipment to it that you want), and the maintenance is much cheaper because you can do it all yourself, even if the airplane is secondhand (except for the annual inspection--you can only do that yourself if you were the primary builder). Homebuilts also tend to have newer, fancier "stuff" than certified airplanes, because you don't have to go through lengthy certification processes to add those things.
On the negative side, homebuilts do not come with that guarantee of performance or safety standards. That doesn't mean that they don't meet them (many do), just that they aren't proven to have done so. They are also subject to varying degrees of build quality--some builders produce amazing stuff, others I wouldn't trust to build a Lego kit properly. Designs with many flying examples (or those from established kit makers) are generally less risky than one-offs; airplanes built closely to plans generally have less risk than those with drastic modifications or unusual engine installations. You also lose flexibility with use; you can't use the airplane for commercial purposes, and your resale market is much smaller--fewer people are willing to buy used homebuilts. And finally (obviously), you have to invest the build time--something that's enjoyable to some, and a nightmare to others.
My dad and I (along with a little help from the rest of the family) built an airplane while I was in high school. I'm now looking to build one myself (albeit a smaller, more affordable one) as soon as I finish saving up the money for the kit.