Some external drive enclosures use a nonstandard interface to the internal drive and/or some level of encryption meaning the bare drive on it's own may not be usable. Best bet is to search for any examples of people disassembling them, sometimes user reviews mention this. It's almost certain that breaking the enclosure apart voids the warranty on the drive and for some enclosures this can only be done destructively (ie no putting it back together and pretending you didn't....something to think about).
As to why they're cheaper than internal drives, I doubt we're getting something for nothing and I also doubt the manufacturers wouldn't drop the prices on the bare drives if they could. It may be the external drives are ones that failed the quality assurance needed for internal bare drives while still being functional, so I wouldn't bet that a salvaged external drive could be trusted in heavier use.
Unless the price difference is great and you know for sure you can pull the drive from the enclosure I wouldn't personally bother. You know what you're getting with an normal internal drive and have the warranty to fall back on. Much less so with external drives.