Well, some have reason behind it. Jasper is a big, buff Quartz, like Rose. So that's why people want them to bump uglies. That's also where Greg x Jasper and to a lesser extent Greg x Amethyst (along with the implications of the little butler incident) come from.
The reasons why Greg x Pearl are a thing also extend to the other Pearls, with the added effect of Pearl being kind of like a sister who's sister is dating a guy she hates/is tsundere for.
Steven x Peridot comes from the redemption arc, similar sizes, the headcanon that Peri is a child gem, and maybe a bit of SI-ing as Steven.
Some people ship because they think two characters would hypothetically make a good couple, not because they'll end up together.