Wouldn't want to overgeneralise on that... Just off the top of my head I can think of numerous significant airports in Europe that are not served at all generously by general public bus routes (IIRC starting with ORY, CDG, MXP, ZAG). (By which I mean non-"airport express" routes or similar; and sometimes even when other routes exist they are often poorly explained/advertised (e.g. CDG, even with a route planner; CPH unless using a route planner) or intended for workers more than travellers (LIS: no non-cabin luggage on buses, and some don't reach main terminal.)