Because you're immediately excluding people on old devices, on underpowered devices, on devices on bad networks, on devices with weird/incomplete/glitchy/limited support for what you need, sites where JS just errors out (and dies, because there are no recovery options if there's a global JS error)... And the list just goes on, and on, and on.

The inability of developers to look beyond their latest-and-greatest dev machines with unlimited power and everything under the sun enabled is just mind boggling.