the cost is not the issue
"My Logitech 2.4ghz mouse has a tiny receiver that sticks out less than 5mm from the side of my laptop and doesn't snag on my laptop bag.
The mouse has an on/off switch to save batteries.
it cost less than £20."
The cost is frankly relevant. for laptops which have a limited number of usb ports, not having one of them clogged by a dongle is worth a lot.
I only have 2 usb ports on my laptop, which means I have 1 usable port because of my mouse.
And before you say, just get a usb hub, that's more stuff to lug around just to connect my mouse, no thanks. For you that might be fine but not for everyone, which is why there is a market for this type of mouse.