Guys like him ordinarily would not be considered unattractive. He had money and was connected to Hollywood through his step dad.
My guess is his awkward personality, unrealistic expectations of having "beautiful women" never reject him, and what can only be described as a "creep factor" vibe along with an absence of genuine warmth made him un-dateable.
There's something about how some men look at women, their vocal tone, and their demeanor that raises "red flags". It makes women feel nervous around them and causes them to look for polite ways to excuse themselves. Oftentimes they can't put their finger on it.
Elliot Rodger thought to himself: "I'm a good looking (nice guy) and rich. You should be all over me!" The true essence of feeling "entitled".
With each rejection he probably became more angry and frustrated with women instead of changing his approach to them.
Some people would rather attempt to change the world than to change themselves.
Their massive ego reinforces they have "everything" a woman could want in a man and yet their lifestyle is one of an "Incel" (Involuntarily celibacy). It infuriates them to see beautiful women with men they consider to be less of a catch than themselves.
Elliot Rodger and guys like him usually have a difficult cultivating friendships with other males as well! They're oftentimes loners.
Jan Lee wrote an article: Debunking the Nice Guy Myth
" A Nice Guy Syndrome is actually pretty simple. It’s essentially this: A guy who proclaims that he treats girls really nicely and thinks of himself as a pretty nice guy and is baffled that he can’t seem to get a girlfriend and thus (blames it on girls) and their obsession with dating ‘a-holes’
They believe that if you’re nice, generous, friendly, and helpful to someone you’re romantically interested in, then that someone is obligated to reciprocate your feelings. The ‘nice guy’ mentality is also somewhat immature since it cannot conceive that a rejection is often not the fault of anyone and usually just a result of incompatibility."
My guess is it was guys like Elliot who coined the phrase: