There are many different kinds of love. People believe you can love several of your kids simultaneously. Even with children the love won't be of the same kind for each of them, rather somewhat different according to their personality, but parents usually love their kids.
Kids love their parents, their grand parents, all at the same time.
Brothers often love each other no matter how many there are.
There is this idea which took hold in society that erotic love can only exist with one person. People still believe you can sequentially love people erotically as people re-marry. Still, somehow there is this idea that erotic love can exist only with one person at a time.
It didn't use to be that love was considered unique, but both marriage became monogamous
and marriage became conflated with love, and together we got to this strange case of an almost total belief that you can (erotic) love only one person at a time. Why did everyone use to believe the earth was flat and that the sun revolves around the earth? They just did because they were told it was so. Why did people believe in the Greek gods and worshiped them? People believe what they are told and live accordingly.