Why can't we use MS Excel itself to generate the reports?

MS Excel was not designed to be a report generation library, so it has several disadvantages compared to XlsIO. Here is a list of some of the problems in using Excel as a reporting component

Microsoft, themselves do not recommend using Excel as a report generation server side component. The reasons are clearly explained in this Knowledge Base article from Microsoft.

http://support.microsoft.com/default.aspx?scid=kb;EN-US;q257757#kb2.

Here is a quote from the article "Microsoft does not currently recommend, and does not support automation of Microsoft Office applications from any unattended, non-interactive client application or component (including ASP, DCOM, and NT Services), because Office may exhibit unstable behavior and/or deadlock when run in this environment."

Speed : Excel automation is several times slower than Essential XlsIO while generating reports.

Cost : Licensing XlsIO is much cheaper than MS Excel licensing options. Here is a link to a Knowlege Base article from Microsoft.

http://support.microsoft.com/default.aspx?scid=kb;EN-US;q243006#kb2

Usability : Essential XlsIO has a very intuitive object model that is modelled after the Excel object model which makes it very easy to work with. XlsIO also includes some additional helper functionalities like the ImportDataTable method which makes programming with XlsIO much easier than using COM Automation. Intellisense comments also make the job of programming with XlsIO much easier than Excel automation.