Problem with async error handling with Express

I am trying to avoid the try catch block in route handlers. I am trying to catch the error in the following manner:

const catchAsync = (fn) => {

    // why this function does not have access to req,res,next?
    // I am passing async(req,res,next) as an argument, hence
    // req,res,next should be known to this function

    fn(req, res, next).catch((err) => next(err));
  
};

and then in the route handler:

exports.createTour = catchAsync(async (req, res, next) => {
  const newTour = await Tour.create(req.body);
  res.status(201).json({
    status: "success",
    data: {
      tour: newTour,
    },
  });
});

Now the problem is that, I don't understand why the function fn(req,res,next) inside the catchAsync function block, does not have access to (req,res,next) when called ? All I understand is that I am passing async(req,res,next) as an argument to catchAsync function. Hence when the function gets called inside the catchAsync block, it should have access to req,res,next