Blame Hipparchus! Think if it more than an eyeball grouping, than accurate photometry by instruments. There are sixteen stars brighter than mag 1 (Hipparchus probably only knew 15, as Greece is too far north to see Alpha Centauri), and they all get lumped into the top slot. Now, I can see that Sirius is brighter than Betelguese (-1.46 compared to 0.5) but my head doesn't go 'it's precisely 6.1 times brighter'. They're both 'bright stars', worthy of note.
What is shocking is the effects of light pollution. When I moved to Scotland I started living in the middle of nowhere, outside (and some distance away from) the bright lights of Aberfoyle. My nearest streetlight was two miles away. The sky was full of stars - easily down to the naked eye-limit. Say 4000, plus the Milky Way, easily visible (inbetween incessant rain and clouds of midges, of course). So many stars that I found it confusing to work my way around constellations: I'd never had such dark skies.
Today, I live in Airdrie, surrounded by streetlights. Magnitude 3 is a great night. Call it 150 stars, tops. The Milky Way? Never seen it here without binoculars or a telescope.