Explicit need for a key name

Why do I explicitly need to set the key name in relationships, even though the tables have appropriate field names. For example

class Post extends Model
{
    use HasFactory;
    //
    public function getpostdetail() // get the post from the post_details table
    {

        return $this->hasOne(PostDetail::class, 'post_id', 'id');
    }
}

works as expected, whereas the following fails

return $this->hasOne(PostDetail::class);

The post_details table does indeed have the post_id column. The same is true for all other relationships, for example, in a Links model I have the following relationship that works:

return $this->belongsTo(Post::class, 'post_id');

Whereas this does not:

return $this->belongsTo(Post::class);

Even though the links table has the post_id column.