I think there are two main reasons here.
First, it's easier. There's a ton of material for writing romance, and most people know more or less what it entails, even if it ends up as the most generic romance you can find. Writing any non-romantic relationships is harder because there's very little tropes and established culture to follow, and relationships like that generally have significantly different dynamics from the typical romantic ones.
Second, romance sells, broadly speaking. A huge majority of people will enjoy a good romance, and while similarly many people will enjoy any platonic relationship in a story, it won't have the same feeling to it unless written really well. There's strong feelings involved in romance, and people are drawn to those.
There's a reason why tragedies, action stories, and thrillers are more popular than pure slice of life, and there are a lot of similarities between that comparison and romantic/non-romantic relationships.