Why are Asia and Europe considered different continents?

There's actually more geological differences between china and india (30,000 ft high himalayan mountains, two different tectonic plates) than between china/eastern russia and europe (both are on the same tectonic plates and the ural mountains of central russia, the commonly accepted boundary between europe/asia are only 4,000 ft high and you can easily drive a car through the little mountain pes to get past the ural "mountains"