Under your system, people with the most GP would always get the most rewards. This is good for them in the short term, but in the long term it is better for the weaker members of the guild to become strong quickly. All the new end game content, which drops way more valuable loot, is centered around the idea that the whole guild needs to work together to succeed and earn equal rewards. With TB and TW in mind, it is much more beneficial to everybody for a weaker guild member to flesh out their teams than for the strongest guy to get slightly better pit raid gear. In the end, would you rather be consistently performing better in TB and TW or consistently getting a few extra challenge gear materials from the pit?
Also, when you ask why else someone would be in a guild other than for the rewards. I can only speak for myself, but getting to know and like my guildmates has made this game way more fun than if I was just playing alone. That's why I like that the game is moving in a direction that takes the emphasis off competing within your guild and is starting to focus more on cooperation and competing with other guilds.