Many Colt parts are produced by DOD contractors that supply most of the AR manufactures in this country. One company I know of produces 30,000 carriers a month and over 20,000 bolts along with 60 something other small parts.
Most parts come from the same places, the problem is other companies use sub standard commercial grade parts, and those companies and parts are better known and more popular with the uneducated AR consumer that purchases the least expensive thing they can find. If you purchase Colt parts you know they will meet DOD spec, Look for other companies that have real weapon(not accessory) contracts with the DOD. FN, LMT.