So the big question: "Why are sleep sacks sleeveless?"
At first you might think: "The baby's little arms will get cold!" But the concept of sleep sacks is built around layering the baby's pajamas inside the sleep sack, kind of like when you go hiking or running.
For instance, if it is cold, your baby can snuggle down in heavier, long-sleeved pjs inside the sleep sack, but if it is warmer you have the option of using lightweight pjs or just a diaper. This gives you versatility so you don't have to invest in 3 or 4 different types of sleep sacks. As well, by being able to vary what goes on under the sleepsack you can ensure your baby is always getting adequate airflow.
So now you know the secret behind the sleeveless!