A lot of people have identities based on loud-but-poorly-thought-out politics/ideologies. Vocal / recent-convert / squeamish teenage vegetarians are hardly unique in this regard, and nor does it mean that intelligent people haven't arrived at similar conclusions for reasons that aren't vapid. As with most things, though, the loud "I believe so hard in cause X" people tend to drown out the thoughtful ones. (Also, the vegetarians I've known have been more pragmatic than the vegans, who are generally more ideological.)

The only group I've seen seriously struggle with the whole "OMG I'm killing bacteria by breathing, what should I do" issue are the Jains, and I find them pretty silly. (I'm not a vegetarian, but when I was...I still found them pretty silly.)

Some vegetarians are amazing cooks, too. Constraints can really drive creativity.