On run of the mill singlemode fiber loss is roughly 0.5dB/km. Splitting off half of the light (and you don't need half, much less is fine), would result in 3dB of loss.

Fiber tramsreceivers automatically negotiate the power needed to sustain the link. So in case of datacenter or city networker, you could do a split-of without introducing too much loss and error rates.

Trans-oceanic fibers are different beast but I guess someone figured it out.