A couple other factors to consider:
Most external drives come with 3-5 year warranties, to give the owner more peace of mind of the durability of a drive that'll probably get banged around in a bag or car seat or drawer. Most internal drives only have a 1 year warranty. So part of the price you're paying for with an external drive is the longer warranty, which you're giving up if you crack open the case in a manner which makes it obvious it's been opened.
Some of WD's external drives don't have a SATA connector. WD designed a custom circuit board which accesses the drive directly via USB, so it can't be used inside a desktop computer.