A text with background is a composed entity: a text and a colored rectangle. SFML provides stuff for both, so I don't know why there should be an additional feature just to put them together. I think it's easy enough to handle on user side, isn't it?
Plus, you usually don't want the background to have the exact same size as the text, in many situations you'll have the background at a fixed size, and then you'll align the text inside it.