Co-channel contention
Nearby Wi‑Fi networks using the same channel can often coordinate access to the medium rather than simply destroying one another's transmissions, but everyone still shares finite airtime.
Overlapping/adjacent interference
Poor channel choices can make neighboring transmissions harder to decode cleanly. On 2.4 GHz in particular, the limited spectrum means careful channel planning matters.
Non-Wi‑Fi interference
2.4 GHz is also used by many other technologies and household devices. Interference can be intermittent, which is why a network may appear fine at one time and poor at another.
Wider is not always better
Wider channels can increase peak throughput when spectrum is clean, but they occupy more spectrum and may encounter more competing activity. In dense environments, a narrower channel can sometimes be more stable.
Automatic channel selection
Consumer routers often choose channels automatically. That can work well, but crowded environments may change over time; a one-time manual setting is not guaranteed to remain optimal forever.