The reason the ISM bands look so "hot" or busy on a spectrum analyzer is exactly because they're a "free for all." It's a classic tragedy of the commons: everyone keeps "shouting" at max volume to be heard and there's no coordination between operators and users of different services.

Here's an example: in a typical apartment block, each resident runs their WiFi access point at full power. In any one apartment, there's a ton of interference from neighboring units. If everyone coordinated and reduced their WiFi power to just what was necessary to cover their unit, it would result in less interference and higher net data rates for everyone. But no one is incentivized to do take action unilaterally.

LTE and 5G networks are very careful planned to limit intracell interference and handover zones between towers. This central planning maximizes the efficiency of the network. It allows your cell phone to connect to a tower 10km away if you're in a rural area, and results in higher spectral efficiency for the overall network by increasing signal-to-noise ratios for all users.

With a more sensitive spectrum analyzer, you'll see that the cellular bands are actually extremely busy. In fact, macro networks are almost always at capacity during peak usage. However, all the devices are "speaking" at the lowest volume necessary to be heard.