It was because there were real hope to find intelligent life in the Solar System itself - as crazy as it might sound now.

Yes. Von Braun wrote an otherwise realistic novel in which earth's explorers find intelligent life on Mars.[1] Heinlein wrote realistically of native intelligent life on Mars and Venus, with far more benign environments then they actually have. But once probes got there, we got to see how bleak they are.

There's a little hope for extrasolar planets, now that we can detect some of them.

[1] https://en.wikipedia.org/wiki/Project_Mars:_A_Technical_Tale