Why is there no resistor in a christmas LED string light?
In my experience, and I've disassembled quite a few of these strings, there is indeed a resistor in one of the LEDs.
Look for a slightly larger LED receptacle, almost always at the far end of the set. If you extract the LED, there will be two brass contacts and apparently no resistor. The resistor is behind one of these metal contacts. Is a tiny -but not SMD- resistor.
In a translucent-wire string that failed, I found one of these tiny resistors in a leg of the last LED, under heat-shrink tubing.
Maybe mine are different, but almost every time I find these resistors hidden in the receptacles. They look like these: https://rayshobby.net/wordpress/wp-content/uploads/2011/08/IMG_0669.jpg except I find them in gray or blueish enamel, instead of beige. They're about 1/3 of a rice grain. Hope this helps.