So it's easy to make a heavy board and hard to make a light one ?

I hear this often, but don't agree with it.

Although it is easy to construct  a heavier  board by adding more material, what most people don't realise is that some shapes don't suit greater weight and some do, also  striking the right balance between weight, buoyancy, length,  and flex is an issue with heavier boards. So, overall designing a heavier board is just as demanding or possibly more demanding than  designing a light board.

For example:

Adding weight reduces buoyancy, so the board cannot be as thin.

Adding thickness to make up for lost buoyancy reduces flex and to some extent control.

Length can be used to add lost  buoyancy, this increases flex.

On the shape front heavier boards require a more rail carving approach to turning than lighter boards and this ideally leads to shapes other than the typical lightweight malibu, these are not well understood by the vast majority of shapers.

Every design element affects every other design element in a board, and weight is no exception.