Couple of things, there are just more and more devices that have high frequency electronics. So increasingly we're surrounded by a sea of radiators.
A problem with all transmitters is they produce out of band signals. Mostly harmonics, but also lesser amounts of other frequencies. And another issue is that receivers only have limited ability to reject out of band signals. A strong out of band signal will reduce your receivers sensitivity. (Difficulty, out of band rejection comes at the expense of ultimate sensitivity, pick your poison)
The problem then is that the amount of energy needed to raise the noise floor is tiny, minuscule. And we have all these devices that all add a tiny amount each, which then adds up to something. If you raise the noise floor a few db it matters a lot, because following reason.
Consider this, if you double the distance from the transmitter, the signal strength drops by ~6db. So if interference reduces your receivers sensitivity from -120db to -114db it cuts the effective range in _half_[1].
[1] It's not quite half because in the real world you have air, trees, and buildings in the way which means your signal drops off faster than the inverse square. Instead the exponent is more like 2.5(outside) to 3.5(inside)
A problem with all transmitters is they produce out of band signals. Mostly harmonics, but also lesser amounts of other frequencies. And another issue is that receivers only have limited ability to reject out of band signals. A strong out of band signal will reduce your receivers sensitivity. (Difficulty, out of band rejection comes at the expense of ultimate sensitivity, pick your poison)
The problem then is that the amount of energy needed to raise the noise floor is tiny, minuscule. And we have all these devices that all add a tiny amount each, which then adds up to something. If you raise the noise floor a few db it matters a lot, because following reason.
Consider this, if you double the distance from the transmitter, the signal strength drops by ~6db. So if interference reduces your receivers sensitivity from -120db to -114db it cuts the effective range in _half_[1].
[1] It's not quite half because in the real world you have air, trees, and buildings in the way which means your signal drops off faster than the inverse square. Instead the exponent is more like 2.5(outside) to 3.5(inside)