Why are there so many buttons in a helicopter?

Because they are such complex machines.

Helicopters must have all the same switches and controls any airplane has, plus the fact that most modern helicopters have 1 or 2 turbine engines puts them at the same level of complexity as a small jet airplane. Helicopters also must give the pilot very high visibility in a wider viewing range due to the way they are used, this results in a much larger windscreen, and much less room for buttons & switches.

In reality, helicopters have approximately the same number of buttons and switches as an airplane of equivalent size, but they must be crammed into a smaller space to accommodate the larger windscreen.