They are just nice people and actually genuinely like the main character and want her to be happy, it's just that the appearance of the demon lord requires them to defeat it and there's a perfectly strong girl there.
Aside from that and having her be a knight for really bad situations, they don't want to use her, they just let her live her life and are always polite to her, in fact, they are pratically the closest thing to parental figures she gets, sadly, their son as we know didn't inherit his parent's kindness, but he will turn around, he never really falls in love for the main character, but he likes her as a friend and apologizes for his attitude.
Also, yeah, having the king and queen always helping her and being nice makes the MC loyal to them just because she likes them, helps that they are genuinely nice, which is troublesome for whoever wants to assassinate the royal family or has problems with them, but that only happens later on.