Because, honestly, who needs to deal with the drama?
You think it's all sunshine and rainbows at first, but then you get stuck in a never-ending cycle of Flexbox Nightmares
You'll be fighting with your designer over the smallest things, like "Is this a grid or is this a flexbox?" It's like trying to explain the plot of CSS Stalkers
Eventually. After weeks of wrestling with it, you'll be like, "Fine, I'll just use Flexbox and be happy with it." Flexbox Converts You to the Dark Side