Because dealing damage in this game has become so vastly simplified, and there are no "conditional" situations to worry about (like, this mob is vulnerable to fire, or that mob has extremely high evasion), if the exact results of the handful of stats that remain were spelled out so clearly, there'd be little or no reason to go after other pieces of gear. If the game did the math for you, and it became obvious that upgraded tomestone hat increases your damage by 2.526 DPS, while raid hat increases your damage by 2.521 DPS, there'd be no reason to go after the raid hat (except possibly for glamour).

But they want you to chase the raid hat. They want you to chase ALL the hats. So, they do their best to obfuscate which hat is BEST hat - and because there's no conditional situations, one hat will always BE Best Hat. Sure, folks will crunch the numbers and figure out the details - but it's a relatively small number of players who actually scour the Internet for that kind of thing. The bulk of players will have no idea, and just chase the stats they THINK are best. ("Oh, boy, with all this skill speed my MNK's gonna punch like E. Honda!")

It's a fundamental flaw in the design of the game. They want folks to keep playing, to stay subscribed, and to participate in as much of the content as possible. However, they've removed most of the REASON to participate in all of the content. Why were folks still farming Sky Gods in FFXI a decade after the content was added? Because in certain situations, gear from there was STILL the best to have.