this is normal behaviour. If you need to provide rich text, you can bind this input variable to a column in your list.
To do this, go to your workflow settings and edit the start form. Then connect your control to a multi line text column of your list. Afterwards you will be able to see the ribbon when entering the text box on the start form. Inside your workflow you can then use this column as required.
Thanks for your reply. But, I found a workaround. I disconnected the control from the variable. So, I got an option to enable rich text for this control. Afterwards, I re-connected it to the workflow variable. I saved the form and published the workflow and I was able to see the ribbon when I placed my cursor in the text box.