they are like 14g or even 21g per serving. They may be low in calories, but too high in Carbs. Why is that?

Most "sugar free" candies are sweetened with a mix of sweeteners, and usually have "alcohol sugars".
The true or "net" carb affect would be the total carbs, minus the fiber (if any)and alcohol sugars.
So, they may seem high in carbs at first, but the "total carb effect" is much lower.
However, some people have problems with alcohol sugars, like a laxative effect. So I do not eat them much and if I do it is 1-2 pieces.

Btw, alcohol sugars, include stuff like malitol and sorbitol.

Sorry, I always have these 2nd thoughts after I submit the post. lol!

The true or "net" carb affect would be the total carbs, minus the fiber (if any)and alcohol sugars.
So, they may seem high in carbs at first, but the "total carb effect" is much lower.

Thankss for the reply! I have a bag of Brachs Sugar Free candies (Splenda) in my hands.The total carbs is 21g, and 0g fiber, and 21g sugar alcohols. Does that mean this bag of candies has a true(net) carb of zero per serving? Wow. I don't think they gave me a laxative effect. If I don't care about the calories (60 per serving for 10 pieces in this case), does it mean I could eat the whole bag because it basically has no carb? You probably could see that I miss eating candies a lot, lol.

Things in moderation I think is the way to look at it.
I personally got the extra weight I had by eating too much, and of course the wrong foods!
Not evryone has a bad effect from alcohol sugars.
But again, I wouldn't eat a whole bag of cany, and I don't count calories anyway.
Good luck and glad I could help.