When you're on a wired connection, only your laptop is making use of that wire - it can send and receive data at 1Gbps without having to worry about what other devices are doing, using an electrical connector whose properties are almost constant.
When you're on WiFi you're sending data on radio frequencies. If you've ever been listening to the radio in a car driving in remote areas, or going through tunnels, you know that the signal can fade in and out - the same issues exist in your home, with walls and ceilings between the router and your devices, furniture and doors (and people) getting in the way, microwave ovens being switched on, temperature and humidity changing.
To ensure you can still get reliable communication as the environment gets more challenging, your phone/laptop/whatever will lower its data rate (trading speed for reliability).
On top of all the above, your devices all share the WiFi radio frequencies (and share them with neighbours as well). If two devices try to transmit on the same frequency at the same time their transmissions collide with each other and are lost. In this case, each device has to repeat its transmission later. As more and more devices use the WiFi, collisions become more frequent meaning more retransmissions, and lower throughput for all devices.
