Someone once compared to raising a child to going to war in that despite whatever preparations you make to deal with it, no one really knows how they are going to handle it until they are in it and it's too late to back out.
Then some people who thought they would like it ended up hating it, and others who thought they would hate it ended up loving it.
Fostering a child is a huge responsibility and not everyone has it in them to do it. If they did, then the only children in foster care would be orphans or whose parents were too incapacitated to raise them. For many people, if you have a kid of your own, then you don't have any choice but to raise them... even if they really don't want to. Willingly taking on another kid when it's not your responsibility is something they cannot see doing. The financial cost of raising a child is very high and they take up a lot of your time with many of your dreams and desires needing to be abandoned for the responsibility of raising kids. Unless you have a desire to raise children, raising someone else's children would be considered an unnecessary burden. If they don't want to do it, don't push them because unless they suddenly discover that they love being a parent, odds are they would not be good to the kid.