First, it is worth clarifying that the pilots don't control the main cabin lighting, and it is actually the flight attendants who dim the cabin lights, not the pilots. The primary reason cabin lights are dimmed during takeoff and landing is safety, and it comes down to how human eyes work. It takes sometimes 30 minutes for eyes to adapt from a well lit environment to darkness, and a human eye can take up to 10 minutes to adapt from bright to dark.
Most airline accidents take place during takeoff and landing, so by dimming the cabin lights, your eyes get adjusted to the darkness outside and you would be better prepared in case of an emergency. The idea is to make the cabin environment similar to the exterior environment.
In case there is a serious emergency during the takeoff or landing and the people have to evacuate through the emergency exits and slides, one of the most likely problems would be that all electrics inside the aircraft go off too. If the cabin lights were very bright, safe evacuation would not be possible, as your eyes would take time to adjust to the outside brightness. Dimming the lights also makes the glow-in-the-dark floor pathways, which point you to the nearest exit, more visible, and people in aisle seats would notice them more easily when the lights are dimmed.
The requirement to raise window shades during these phases is connected to the same safety logic. With blinds raised, you can see potential hazards outside, and you know not to open a cabin door if there is a fire outside it. The passengers sitting at window seats can help other passengers and cabin crews by informing them in case of extreme emergency situations like engine fires. If there is a rescue team outside the aircraft, raised shades also help them see whether any passenger is there inside the aircraft.
As for seats being in the upright position, if there is an emergency situation and you are pushed forward, having your seat reclined means your head has more distance to travel before hitting the seat in front of you, resulting in more injury. In the event of emergency, a reclined seat will also create an obstacle for the person sitting behind you while evacuating.
It is worth noting that there is actually no regulation requiring blinds to be raised or lights to be dimmed during takeoff and landing. It is up to the operator of the aircraft or the airline's policy, though it is pretty rare that this format is not followed.