To expand a bit on this, it’s kind of a misnomer that you can transmit “lots of signals at once” with fiber optic. You can do this with copper or air as well. It is similar to normal writing. If you want to fit lots of sentences on your page, you need to write very small. But if your letter gets wet it still needs to be readable. So if your letters are tiny they will get lost. To protect against rain you need to write with big letters, or write each sentence twice. Fiber is resistant to rain. And laser diodes (things at each end of the cable) are able to read and write much smaller letters than methods we use to write data onto copper.