It's a simplification for playability
While the Monster Manual on 314 says that a Large (long) creature's maximum length is 16 ft., it also says its minimum length is 8 ft. Thus, generally, size categories are flexible, and making size categories flexible simplifies play. That is, rather than having extremely granular size categories, it's easier and more playable to assign creatures a size category that's close enough and make exceptions if such exceptions are warranted. For example, the typical Dungeons and Dragons 3.5 horse has a size category of Large, yet the typical real-world horse is only 8 ft. long, littler than its 10 ft.×10 ft. space (plus its 5-ft. reach!) would indicate.
Also keep in mind that a creature's space as listed in its stat block isn't a really good indicator of the creature's actual, real-world dimensions. A complicated creature with wings and a tail like a chimera, manticore, or wyvern—were an examiner to spread out one of these creatures, Vitruvian Man-style—would probably occupy a much bigger area than 10 ft.×10 ft., yet such creatures are nonetheless treated as Large creatures with 5 ft. reach in combat because that's easier for the game… even if doing so occasionally threatens verisimilitude.
A DM that decides to go through the various monsters and pick which long (as opposed to tall) monsters should have greater reach than normal should be aware that monsters with reach are typically much more dangerous to PCs than monsters without. For example, a house rule saying that the 15-ft.-long wyvern has a reach of 10 ft. instead of 5 ft. makes the wyvern a much more formidable foe.