Hey Deepthi -
Please keep in mind that Tableau is a Visual Analytics tool - we don't try to solve the same problems as Excel.
Therefore, features that exist in Excel may not appear in Tableau, and vice-versa. Tableau is generally not an appropriate solution for showing lots of columns and rows - just use Excel or an "old school" report tool like Crystal Reports or SSRS for that work.
To answer your question more directly, Tableau doesn't support collapsing columns in the same way Excel does.
Thanks for your response, we are locked down to using tableau so I will need to find an alternate approach.