That’s because they’re utopians.
By definition, utopians want to live in the past, not the present or the future.
They want to, as the song “Woodstock” put it, get themselves back to The Garden (of Eden, which depending on your point of view, either existed a very long time ago or never existed at all.)
At least ditzy people who fantasize about getting married in Disneyland wearing full princess regalia are dreaming about going to a place that actually exists in the physical realm, no matter that its business is fantasy. (Note that these are the sorts of individuals sophisticated “progressives” most enjoy making fun of.)
Leftists delude themselves and others by calling themselves “progressives,” just as they insist on twisting words like “marriage” to mean what they want them to mean.