Markets aren't efficient in any space ;)... (review the assumptions required for an efficient market, so see how reality violates all of them with consequences, or just look at the empirical, historical data; also see behavioral finance literature).
All existing investors/shareholders are diluted in new rounds, almost always. And all shareholders -- including investors -- in high-tech startups are minority shareholders (less than 50%), with the exception of single founders early on, typically. (Minority shareholders, investors especially, still have certain rights.)
I hope these notes help!