I don't think there's a particularly simple answer to this. By the late 40s, the rocket ship -- usually based on the shape of the V2 -- was ubiquitous, to the point that by around 1960, some libraries would indicate that a book was SF by putting a rocket sticker on the spine. Winston used: In effect, a rocket became the icon that said "SF". (Valigursky was especially known for his elegant rockets and did many covers in the 50s and 60s, but he was by no means alone. Nor did he limit himself to things that looked like V2s!) Here's one from 1953 by Richard Powers:
Artists wanted to do more and art directors wanted more, and needed to have their book covers simultaneously say "this is SF" and "this is something new and different", and one solution was to have a more fanciful spaceship on the cover. For some reason, this was especially common in the UK where artists like Rick Foss painted spaceships which couldn't possibly be intended to be real.
So my best answer is that you see the result of seventy-five years of artists and publishers collaborating to creatively use the rocket (or spaceship, these days) to signify SF and sell books.