Why I cant draw on panel but I can draw on form itself with this code?

List drawingRects = new List();

        public class DrawingRectangle
        {
            public Point Location { get; set; }
            public Size Size { get; set; }
            public Point StartPosition { get; set; }
            public Color DrawingcColor { get; set; } = Color.LightGreen;
            public float PenSize { get; set; } = 3f;

        }

        private void panel1_Paint(object sender, PaintEventArgs e)
        {
            if (drawingRects.Count == 0) return;
            //e.Graphics.SmoothingMode = SmoothingMode.AntiAlias;

            foreach (var rect in drawingRects)
            {
                using (Pen pen = new Pen(rect.DrawingcColor, rect.PenSize))
                {
                    e.Graphics.DrawRectangle(pen, new Rectangle(rect.Location, rect.Size));
                };
            }
        }

        private void panel1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                DrawingRectangle rect = drawingRects.Last();
                if (e.Y < rect.StartPosition.Y) { rect.Location = new Point(rect.Location.Y, e.Y); }
                if (e.X < rect.StartPosition.X) { rect.Location = new Point(e.X, rect.Location.Y); }

                rect.Size = new Size(Math.Abs(rect.StartPosition.X - e.X), Math.Abs(rect.StartPosition.Y - e.Y));
                drawingRects[drawingRects.Count - 1] = rect;
                this.Invalidate();
            }
        }

        private void panel1_MouseDown(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                drawingRects.Add(new DrawingRectangle()
                {
                    Location = e.Location,
                    Size = Size.Empty,
                    StartPosition = e.Location
                });
            }
        }