Why are extroverts so permeable to outside influences?

I mean in the sense of a group of friends. I have often observed amongst extroverts that it takes one noxious, pernicious individual to infect the entire group who would be just fine without that person. It is a similar idea to the notion that it takes one bully in the workplace to change the healthy, mature dynamic into a dysfunctional, puerile one. With bullies they may be charming to certain members of staff, fawning and ingratiating, building alliances with others and targetting others yet again instead of letting everyone get along in peace. What I have often witnessed is that decent people start to change around the bully either adopting their mannerisms, attitudes, principles or standing by them silently..I personally find people that just readily accept and praise the bully dull witted and spineless but alot of people are like this. I have observed it enough times. I don't care what they try with me, or what they say to me, I stand principled, always.