Because it's just so convenient to have to plug your laptop in via an HDMI cable and have it permanently tethered to your TV, and I just love sitting on the couch attempting to move a basically nonresponsive laser mouse on the furniture surface.
Cost of chromecast+remote: $30
Cost of secondhand laptop + wireless keyboard/mouse: $250
Cost of chromecast+remote: $30
Cost of secondhand laptop + wireless keyboard/mouse: $250