To add thumbnail to post, do the following:
WordPress Dashboard -> Posts -> Add new Post -> Add Media -> Select Attachment Display Settings -> Insert into post -> click on image and edit it -> Set Display settings accordingly.
This does not create a thumbnail for the post Excerpt.
I think what is being asked is, why do the post Excerpt for TT not automatically create a thumbnail for post Excerpts, this is standard with any template.
And I for the life of me, cannot get a thumbnail to show with the post Excerpt without setting featured image, which then puts an ugly looking thumbnail above the post title on the post page...this is ridiculous!