The Tasks report does not include summaries (from task roll-up or manually defined summaries) to enable the resulting spreadsheet to have further analysis performed, e.g. adding formulas. If it included summaries, some of the numerical values would be represented twice or more.
There is a separate report for the Summaries, but has slightly different column headings, as resources are not rolled up in summaries.
To get summaries and tasks in the same report, use the "Grid Report" which will export both summaries and tasks from the grid to the same spreadsheet.