Why are power losses in fiber-optical cables so high?
Fiber optics are used to transport data, not to transport energy.
Compare to radio transmission: 100 dB is common. So the transmitter pumps kilowatts into the air, the receivers receive only a minute fraction. Useless? As power transmission, yes, but that is not the goal.
For fibre optic transmission, an input power of (10's of) milliwatts is normal. The cost of that amount of power is neglectible. The receivers can still reproduce the data, that's what it is all about.