Why isn't there a single start button for all systems on aircraft?
Good question, actually the history of aviation teach us that: any technology on board is robust and capable to not crash or reboot for nothing. Behaviour you can see sometimes on brand new technologies. So everything which is on board is ten years old technology.

In the same time, the pilot needs to be able to switch off or on some systems if they are failing, if you automated all the sequence it start to be difficult to know if the pilot has the real control of the system.

It is only matter of safety and control. Because we already know that we are capable to automatically take off, fly and land UAVs. Why not in an airliner? Because safety. It has to be proven for a long time to convince safety agencies to make the step forward.