Because those costs grow orders of magnitude as they're externalized. $1000 spent during development can offset $1M in energy wasted by a million users. So they should want to pay those costs as responsible members of civilized society.

And if they don't want to, they should be forced to by said society.

To make a quality product and offer a better user experience?

Users may not notice the slowness right away, but if they run a bunch of applications at once they probably will, and they're more likely to notice it while running on battery.