Well, first because I don't think that an optimized character is necessarily one or two levels better. Your natural talent doesn't mean that you get to start at 3rd level, although it might mean that you are as good as a 3rd level character at some things.
Fairness comes in many approaches. Everybody rolling dice means everybody has the same probabilities, so that's fair. So is a point buy system.
It's true that somebody who is born with great strength is probably more likely to gravitate toward physical activity. But they don't always do that. Sometimes they go with something they love, even if it's not what they are best at. More importantly, when you're rolling for stats, you more frequently roll a character that could be decent at a couple of classes, and you need to make a choice. In addition, you still get to add your racial modifiers (I use a version of the variant human that allows you to add up to a +3 divide as you'd like). I think that's reasonably representative of somebody putting their focus in a certain direction for the first 15 to 20 years of their life.
It's not that people won't optimize. But the chances you'll get a perfectly optimized character is pretty slim, though.
I still do S, I, W, D, C, Cha, so what would you do with these characters I'm rolling up right now?
11, 10, 11, 12, 15, 14
14, 14, 8, 13, 14, 15
12, 8, 11, 17, 15, 9
12, 10, 10, 11, 14, 10
15, 14, 11, 8, 9, 11
Of course, you'd have to meet my requirements too:
Bard: Dexterity 10, Charisma and Intelligence 13
Cleric: Wisdom 10
Fighter: Strength or Dexterity 10
Paladin: Wisdom and Charisma 13; Strength 10
Ranger: Wisdom and Constitution 13; Dexterity 10
Rogue: Dexterity 10
Sorcerer: Charisma and Constitution 13; Dexterity 10
Wizard: Intelligence 13
You can probably guess which classes are more or less common in the campaign, and which ones that people are excited about when they roll one up.