Fewer people want to foster than to have their own kids?

For a foster parent you only need to find someone willing to adopt a child, which already register themself into a list. For marriage you need to find someone willing to marry and to marry you out of random people.

Also, marriage doesn’t imply children.

Depends, but I see what you mean.

Never "depends". Marriage doesn't guarantee children, nor even the desire to try for them, both of which can occur without marriage.