In some of these locations you can at least go to the beach, miander around and have general chit chat with others in a similar situation. Enjoy the sun, have a swim etc etc etc.
In the city you can do none of these, it isn't nice to walk around as it is so dirty, noisey and polluted. If you want a swim most have to go to a hotel pool, and well there is no beach.
These coastal towns of Cabarete, Sosua and such like have a completely different offering and you can get by just hovering around and being part of the community.
I live in the capital and I am going to surf, at the beach, right now. It's a 35 minute drive to get there but it can done. How many times can you go to the beach before you desire something a bit different?
You're right, we do not have a beach directly in front of Santo Domingo. Our closest beaches are 25-40 minutes away but they can easily be reached. Besides that, I'll run down some other things I can do. I can go to the largest zoo in the Caribbean, walk around our Botanical Garden, meander the streets of the Colonial Zone and have the same general chit chat you might have in another city. While I'm meandering, I can visit the first street, church, university, monastery, hospital, and stone home of the new world. Maybe pop into the Amber Museum, Larimar Museum, Diego Col?n's house or even the Museum of the Casas Reales.
Outside the Colonial Zone I can stop by the Plaza de La Cultura for the Museum of the Dominican Man, Museum of Modern Art or the Natural History Museum. These cannot be compared to museum's in the first world, but they exist here.
If I feel like shopping I have several malls to choose from: Acropolis, Plaza Central, Bella Vista, Coral Mall, Diamond Mall, soon to come (Blue Mall). If I feel like eating, it's not a decision of what restaurant, it's a decision of what cuisine I want. This is a huge difference.
In all honesty, I don't feel like getting into an argument about how there is more to do in Santo Domingo than anywhere else on the island, although I believe that. I have my gripes with the capital as well. The bottom line is the preference of the individual. For some, they would go crazy with the capital's traffic and fast paced life, others love it. Some may be content simply chilling at the beach everyday, some would be bored out of their mind.
The simple fact is that there are more options in Santo Domingo. Back to the OP's question, you just have to ask yourself what you want out of the place you live.