Do you mean 5Ghz networks (5G is cellular tech, after all)? If so, 5Ghz can travel through walls, but it doesn’t travel as far, because there’s an inverse relationship between range and channel width. Also, 5Ghz has a shorter wavelength; some of the signal’s light will get absorbed by the walls, but not all of it.
Ultimately, you’d still have the same problem: too many radios sharing a limited range of frequencies on a band would interfere with each other if sufficiently close.
It would be akin to having everyone playing different music at full volume on their own personal speaker; you’ll inevitably hear the people closest to you. Radios can’t “hear” anything outside of their chosen frequency (channel), but if other people nearby are also on that channel, you might catch or lose some unintended packets, triggering a resend event (TCP) or causing stuttering/lag (UDP).
The number of channels available for 5Ghz varies by country, with the EU having the most, iirc. In the US, if you try to force your router to use one of the blacklisted channels, your devices will likely not connect (unless they were directly imported), despite being able to use the 5Ghz spectrum.