We've tried to do it on the ground for decades to little success. We give them money, and they say, oh so sorry we spent it all and did 0.001% of the requirements.
SpaceX and these other LEO constellations can do it in a few years, at their own risk.
Also, these constellations serve areas that fiber will never serve. 5g towers maybe in many but not all, but never fiber. They also serve areas like the ocean, and air, some of which will never be served by 5g.
edit: Also there's no risk of trapping us on earth with something like Starlink. It's too low and will resolve any mass destruction in a handful of short years. The risk comes with something like Eutelsat One Webs constellation at 1200km that will last millennia in a Kessler Syndome type doomsday scenario.