I get that a lot, especially from well-meaning older folks with grown kids of their own: "Foster kids just need the same thing as every kid needs: they just need love. That's all."
Short answer: nope.
Think about it like this: if I had a child who fell out of a tree and broke his leg, and I decided that all he needed was love, I'd be a terrible parent. A kid with a broken leg needs a visit to the doctor, an x-ray, a cast, and a pair of crutches at the minimum. He might also need a visit with a specialist, some surgery, a wheelchair, or some physical therapy. Yes, this child needs love. He also needs specific interventions to address the damage to his leg.
Foster kids are not in the system because of some single tiny problem. Especially where we live, there has to be clear threat of imminent harm to children before they are removed from their homes. Because of this, any kid in the system had experienced some pretty terrible things. Those terrible things, unfortunately, cause kids brains to develop in problematic ways.
For a comprehensive overview of how neglect and abuse influence child brain development, here's a brochure from the federal child welfare agency. Briefly, though, inconsistent care giving, neglect, or abuse creates an environment of toxic stress. Children can respond to this stress by becoming hyper-alert, easily upset, or over-sensitive to sensory or relationship stimuli. Alternately, children can respond to this stress by withdrawing and becoming under-responsive to their environments. They may not develop the ability to form strong relationships. With inconsistent or inappropriate input, children develop unevenly, fall behind their peers, or fail to meet developmental milestones.
To put it bluntly, the situations that land kids in foster care are situations that cause brain damage. Yes, a foster child needs love. He also needs specific interventions to address the damage to his brain.
Love is necessary, but it is not sufficient.