Indeed, when selecting a range of cells, we set the active cell to the top-left cell. In theory it could be any cell inside the range (it doesn't need to be the bottom right, but it does need to be inside the range).
The reason is just to keep the API a little simpler. There are already kind of "too many" options supported, like you could save many ranges, say select with ctrl the cell A1, B5 and E3. Or have different selections in different panes when you split the sheet. It all makes the API kind of complex, and I am not sure anybody is using that stuff anyway.
But we could add the option to have the active cell be inside the selection, not the top-left, if you think you can need it. What would be the use case for it?