Why is yarn not sold in balls?
Balls are not a practical or economical shape for machines to roll. Yarn is sold in skeins or hanks, because that’s the way the winding machines are configured. Balls also prevent you from working from both ends of the yarn, which is sometimes necessary.
Can you knit straight from a skein?
Frankly, if you can wind a ball of yarn by hand without a swift or a ball winder, you can knit directly from a skein. The only difference between the two is winding the skein up between knitting sessions.