Why doesn't Tor Browser just spoof a default screensize? Why is it necessary to tell websites what size the screen is? Would it be possible to still see respons

Why doesn't Tor Browser just spoof a default screensize? Why is it necessary to tell websites what size the screen is? Would it be possible to still see responsive websites if a default screensize was chosen? Perhaps a few 'presets' could be created for different aspect ratios and then it would alright? Just curious I guess, it's a shame that there are people out there who've likely done everything right except changing the size of their window, and maybe they got v& or even hurt because of this.