Not a dumb question at all! The main reason is because there is much more land mass in the northern hemisphere than in the southern, and land tends to heat up more than ocean. You can actually see how this effect balances out on this site: https://climatereanalyzer.org/clim/t2_daily/
If you look at the annual temperature range for the northern hemisphere (NH) it has a range >10C, while the southern hemisphere has a range ~5C.