"Can someone explain to me why there are so many companies selling AK "parts kits" (as opposed to complete rifles)?"

it's simple. about 90 million ak's have been built since 1947. an everyday, in over a dozen factories in as many countries, a few thousand shmucks crawl out of a warm bed to go to those factories to crank out more.

with 'x' amount of commie countries with a belly-up ecnomy, and only al's to trade for hard currency...plus the gazillion un-needed and obsolete model out there...it's a no-brainer. they HAVE to sell a lot of the new ones as parts, chop up the old ones for parts and scrape out even the worthless ones as parts.

"Is the idea to get a receiver somewhere else, and then build it into a complete rifle by adding a parts kit?"

yup. plus, add 4, 5 or 6 american parts as legallity dictates.

"Or it is an interchangeability thing, like buying a different AR upper to put on your AR lower?"

well, yes...but not primarily. spare parts are cool, but spare rifles are way cool.

"So where do most people get their AK receivers then?"

hesse recs. are 4/$100 at wholesale guns & ammo, for example.

ohio ordnance works sell the ak-47 and ak-74 styles.

global trades has feg recs.

etc.

"So......which OOW receiver would be the correct one to get for a '74?"

they now manufacture and sell a ak-74 specific model with the correct size magwell opening.