You guys are looking way too into the lore reasoning of this when the Doylist reason is readily apparent: to minimize the ickiness.
The reason the novel clarifies he’s a clone is because if he were Palps’ biological son, Palpatine would have had to have banged Rey’s grandma sometime during the late Empire, which is a gross thought we don’t need some EU writer detailing for us down the line.
Meanwhile if he were more clearly spelled out as Palps’ clone in the movie, we would have a flashback where a digitally-de-aged McDiarmid is coddling the young Rey, which would be just as horrifying and also create the unnerving idea that biologically Rey is actually Palp’s daughter.
The sloppy mixture of movie and book explanations we got was ultimately a mercy. He’s a test-tube baby and not the result of wrinkly Sith love-making, but he’s an imperfect clone who’s genetically-speaking more like a son than a literal second Palps.
Considering Palpatine didn’t need to return, in the first place, and Underworld would have had a woman breaking Palpatine’s heart in the time period of her father’s conception… there was still no need to go about this silliness.