There are multiple reasons for WiFi interference, but the most common reasons are:
AC Adapters - This can also include battery chargers. Many chargers and AC adapters are made cheaply and radiate wideband noise.
Bluetooth Devices - While modern Bluetooth devices are designed to avoid active WiFi channels, older devices intrude on the same 2.4 GHz spectrum used by Wi-Fi 802.11b, 802.11g, and 802.11n.
Toys and Gadgets - Many recently-manufactured toys and gadgets use WiFi. This can include drones, remote-controlled devices, display mirroring devices, keyboards, computer mice, speakers, wireless microphones, and more.
Hackers - Hackers can use illegal, interfering devices that can cause interference on WiFi networks.