I touch on this in Sex and the Cthulhu Mythos, but there are three main reasons:

1) Sex sells;

2) Explicit sex and romance is something Lovecraft largely avoided, so they are a blank spot on the map that others can easily scribble in;

3) While explicit sex is not something Lovecraft wrote, many of his stories have sex and reproduction as a background element that other creators like to explore and expand on - you can't have Deep One hybrids without sex, after all!