Because that's the way it works and there is probably another strong reason
The <span>lines from "The Love Song of J. Alfred Prufrock" that most likely influenced Sandburg’s poem are the following:
"</span><span>The yellow smoke that rubs its muzzle on the window-panes,
Licked its tongue into the corners of the evening,
Lingered upon the pools that stand in drains,"</span>