To clarify some of the points above, consider this annotated image of the innards of a generic/cheap USB 'charger':
Note that this entire thing is always 'live' as long as it's plugged into the outlet. Nothing physically disconnects the circuitry from the input when no load is connected. Both the input and output capacitor constitute a small leakage current. Further leakage occurs by design as the SMPS controller remains turned on and will keep the output capacitor topped up, for which purpose it needs to monitor the output voltage through a small resistor devider that also creates a small minimum load that the controller needs to retain regulation.
Note also that in terms of safety, there's barely anything going on. The most important good thing is that the transformer offers galvanic isolation from the grid, which means that if you drop your connected phone in the bath tub you're sitting in, you won't die (note that this is different from the even simpler reactance-based adapters I also mentioned earlier - these are very likely to kill in this scenario). The MOV current limiter (consider it a resettable fuse) in principle should work OK, but the actual safety it provides depends on the quality of the part itself and how it's implemented. It appears to only protect the primary winding of the transformer, which means it'll do jack-excrement in case the HV buffer cap fails short (see below).
The electrolytic capacitors on both input and output are subject to aging. How long they last is a total wild guess; they may poop out after 5 years, or they may survive for 50. You never know what you're going to get. Once they go, we can only hope they do so in a gentle way and without creating a short on the primary, high-voltage side. No guarantees on this whatsoever. They generally pop or fizzle, spewing caustic electrolyte all over the place, which will then start to degrade all components it touches. If the input capacitor would somehow fail short, you're in trouble in any case as this can easily start a fire (there's nothing that will limit current; the inrush current-limiting resistor isn't designed to handle a dead short and will simply carbonize).
Most of the safety in this thing is down to the black-box safety features inside the SMPS controller. How well these work, is anyone's guess. To take a parallel from linear regulators: these generally have safety features like current limiting and thermal shutdown, but they're also known to fail in violation of their own specifications, and fail short (instead of open) frying everything downstream (I've actually seen this happen on my own workbench).
Again, I admit that I leave my chargers, computer, IoT devices plugged in pretty much all the time. But that doesn't make it a smart, sensible or safe practice. As shown above, the safety measures involved are flimsy at best.
If you think your genuine Apple charger is any better: well, it is, but marginally so. It suffers from many of the same inherent problems as the $1 device shown above, since they're conceptually very similar.
The main difference between the device above and USB-C/PD adapters is that the SMPS circuitry is more advanced and there's additional circuitry that manages the communication / power negotation with downstream devices. None of this means they're inherently safer than what's shown above.
Overall, the reality of the market today is that we're not really prepared to pay for real safety features, which would make these devices more bulky and more costly.