Because brilliant engineers can make brilliant technology, but from their perspective, the user will look like a stick figure – just another label in a complex system, just another part in a big machine.
The above image is taken from Google OAuth 2.0 page to access Google APIs.
But when you want to create a good experience, you need to have people who will care about how and what the user will see. What he or she will feel. People for whom user is not a label, but a living, breathing human, in all the detailed glory, with all the the mistakes and biases we carry.
You need people who are verbal, who can converge ideas, but you also need people who are visual, who can intuit, and live with detail, without reaching conclusions.
And then, you are more likely to build a product users will love.