The reason comes from the Shannon theorem: C=B*log2(1+S/N). C is capacity (what we want), B is bandwidth, S/N is the signal/noise ratio.

With modern technologies, we are very close to that limit. Both 5G and satellites are using among the highest radio frequencies the atmosphere can carry, the signal power is limited by electrical power requirements and emission power regulations and there is not much we can do about ambient noise. Fiber uses light which, in theory, can carry many orders of magnitude more information but the problematic is different.

But the thing is that this capacity is per channel, and the narrower the channel, the more we can pack in a given area, and that's where satellites are at a huge disadvantage. Satellites, even Starlink satellites, are really expensive, and they are 100s of km away at best. It means they can only project wide beams, even with they fancy beam forming antennas. Ground based antennas can cover a much smaller area and are much cheaper, as a result they have more channels and therefore more bandwidth. Satellites may prove effective in sparsely populated areas, but not in dense cities.

Fiber, and even copper wires are the extreme case. The channel is a single wire, you can pack hundreds in a cable you can hold in your hand. Compared to wireless transmission, bandwidth is essentially unlimited.