I think in this particular game, being KOTOR 3 more than a Star Wars MMO, many people have never played in a multi player environment before. That type of player is less likely to group, due to performance anxiety, being worried they might make a mistake and be called out/ridiculed by others.
Then there is the other side of that coin, which is where someone like myself resides, where having played many MMO's over many years, having grouped with every kind of personality out there and having grown somewhat cynical of MMO gamers, I no longer have the desire to deal with random folks very often.
Also, with all of the instancing and sharding, players seem to be isolated from one another quite a lot, regardless of overall server populations. If you aren't seeing many other players, that's one more opportunity to meet a random individual, who you may have ended up grouping with, that's out the window.
Add to this, SWTOR really doesn't require grouping, and I think you can understand why many choose to go it solo.
That being said, I do wish there was more of a sense of community.
To the point of the OP, I do agree there is a segment of any MMO population that fits your description. They are known as narcissists.