because love, romantic or not, is something that's always present in human lives (or not present); it's something that evoke a lot of emotion, which is something song writers usually need to write a song.
The same reason some sort of romantic plot is tagged onto every goddamn non-fantasy (and even some fantasy, which annoys the shit out of me) story out there, in whatever medium of choice.
In pretty much all human media from any culture ever, there are three major motivators. At least one is always used to drive the plot along, or at least a character arc along. The three are: beliefs (political beliefs, moral values, religion, whatever), revenge, and love. No matter what human culture you take from, those three are prevalent themes in every single medium.
Love is ingrained in the human psyche. This makes it easy to relate to for all people--for the artists, that means people will relate to them and everyone will feel happy. For the executives, that means they are guaranteed an audience and therefore, money.
Love is one of the more dramatic things people experience, and lets be honest, most of us first-world people don't really experience anything terrible in our daily lives , so they got nothing better to sing about....and also, some people don't have very meaningful passions or even lead their own personality, lol. They think romance will spare them of all the voids in their life, there I noticed, are many clingy songs....(like some oldies on my work station who make people sound really desperate and stalkerlike)
For example, some people sing about nature or subtle topics (and are far more pleasant to listen to sometimes).
Also I guess because it sells. Back to buying people by singing them what they want to hear, right? boy bands sing to girls and woo them, some woman can go with terrible voices but have the ideal appearance, so men buy them, heavy music sells to angry people, etc.
The industry is pretty corrupt if you ask me...I don't even enjoy the celebrity gossip stuff that much..it's so ridiculous.
A lot of the songs that I listen to are about love, whether it be about the happiness it brings or the sadness and anger that results when love does not work out. I always think of it this way; Music, and art in general, often express one's emotions and can be cathartic for the creator. If you have strong feelings related to love at the time, you have a strong influence to fuel your work, and it often, but not always, results in a particularly exceptional product. Love gives you something to write about, if nothing else will.
It can either lead to a sincere song where you feel that the singer MEANS what they say... Or a forgettable bland piece of commercial crap that is so manufactured it almost has a tag on it, where the singer uses the words "baby," "oooh ah ah!" and "Oh yeaah!,"
Or in the case of Enrique Iglesias, blatantly telling the woman that tonight, he shall be having intercourse with her, regardless of how she feels, it is also a love song.