You need to copy/paste the text of the formula, not the cell.
- Double click on the cell to bring up the formula.
- Select all the text of the formula (I use CMD A keyboard shortcut).
- Copy (CMD C).
- Hit ESC to get out of the formula editor.
- Click on the destination cell.
- Type an = to bring up the formula editor
- Paste (CMD V) and hit Return.
It looks like a lot of steps only because I broke it down so far. It is actually pretty easy and quick.
If you would like to move the formula from one cell to another (which might be your end goal, I don't know),
- Click on the cell
- Click and hold on the cell
- Drag it to a new location and drop it
It seems there should be a way to do what you are requesting using drag and drop but I don't know how. You can copy (not move) it to another cell if you hold OPTION before dropping it but it adjusts the formula. I haven't found a key that copies it without adjusting it.