Your cellphone's 4G connection might be fast enough to handle a 4K stream, but there isn't enough RF spectrum to handle 500 devices in the same block trying to stream 4K. That's why you still need physical internet connections.
If I option-click the wifi icon on my laptop, it says "NSS 2" -- that is, the Number of Spatial Streams is 2. My laptop and router have identified two different spatial paths between them and are using their multiple antennae to send different data down the different paths.. NSS=2 is the limit for my router because it only has 2 antenna, but those routers with a ring of antennae that look like they were designed by the Dark Lord Sauron to top a tower in Mordor can fire correspondingly more simultaneous spatial streams. Cell towers with antenna arrays can do even better.
There is plenty of spectrum, just not for old single-antenna tech.
Doesn't that use 2x the spectrum? If so that doesn't really solve the problem. Sure, you're getting 2x the speeds, but also using the 2x the spectrum which is still a shared resource.
The same trick works for "virtual antennas" built out of phased arrays of real antennas, except you can steer them in microseconds and without any motors. MIMO leverages this.
On the other hand, whenever you run a length of fiber, you create a whole new spectrum which is available 24 hrs a day.