It is also noteworthy that B. splendens still have individual territories in the wild, so its not like they live in a big happy shoal. There are many different kinds of betta as well, but of all the species, we pretty much chose the most territorial/agressive of the bunch to breed for fighting purposes. So while bettas may interact with one another in the wild, it is still probably more of a "get away from my territory" or "let's mate" sort of deal. They are just a bit more tolerant of one another than those we've bred to be hyper-agressive.
And, of course as others have said, bettas do not get lonely. They get bored and understimulated, but suggesting a non-social species gets lonely is contrary to the very nature of the beast. Think of non-social mammals; the only time we really ever see them crowded in with others of their kind is when they were raised in unnatural captive situations, and even then it is usually safer for a human to be their source of interaction since fights can and do break out. And that's without the massive tendancy for rabid territorial agression that bettas have been selected to have.
The best thing you can do for a "lonely" betta is acknowledge that it is in fact understimulated, and make its environment more enriching. Different arrangements of the tank, new objects in and outside of the aquarium, a varied diet, the ability to view other fish from the safety of the tank so that it can perform natural patrolling behaviors, etc. would all help.