I was reading about it the other day and found out it involved all the great powers of Europe and their colonies (which was most of Asia and Africa). So why isn't it regarded as a World War? Due to the comparatively less number of casualties or it something like that?
/u/elos_ wrote two fine answers about why the First World War was truly the first global conflict here and here.
/u/bacarruda also answered this question here.
See /u/HarryCochrane's different view, where they view the Seven Year's War as a world war [here] (https://old.reddit.com/r/AskHistorians/comments/a8okdo/wouldnt_the_7_years_war_be_the_first_world_war/ecfht1s/?st=jsy3d5tq&sh=87595e23).
More can always be said but these are some good places to start :)