why don't serial lights get dimmer..?
Christmas lights don't exactly "share" the same voltage on each lamp - they happen to have approximately the same voltage but that is because they are all in series and have roughly the same impedance to current when glowing.

If your supply is 240V ac and you have 20 lamps in series, each one will have approximately 12V across the lamp terminals. It makes sense for each lamp therefore to be rated for 12V operation and of course if you tried to put that across 240V it would instantly fail and blow.

If you took a 240V lamp and put it in series with another 240V lamp, each lamp would be receiving 120V and therefore the brightness would approximately reduce by a factor of four.