Because it adds way more than it takes. All other quirks are reasonably balanced, but not so Gifted.
- You trade skill points for stat points, but since stats make up part of your skill ratings, your skills immediately go back up a little already.
- Imagine putting all those extra points into Intelligence... You'd get such a boatload of bonus skill points that you'd reach a break-even point before you are well underway.
- Stat points kick more butt than skill points.
- You don't need every skill (nobody needs three different weapon skills, for example). You do need every Stat. Some more than others, depending on your character theme, but all of them matter.
- You get loads of skill points to distribute each time you level up. More than enough for the skills you actually care for, so who cares they start out a little lower? Not so with Stat points; they're hard to come by.
When generating a character, I'd choose Gifted without even thinking about it, then I was in for a hard time choosing the second quirk.