Well, part of that is simply that KDF and Romulan ships simply do not HAVE the weapons people want to equip on them. There are, for instance, practically no examples of a Vorcha blasting away with broadside-mounted beam arrays on the screen because they simply do not have the capability to do this on the show. STO's "cart before the horse" approach of equipping weapons, then deciding what hardpoints to emit them from, rather than giving hardpoints, then letting players decide how to fill them, results in rather awkward-looking behavior and limits the opportunities for ships to distinguish themselves from each other.
Also keep in mind a simple fact,... Trek ships have ventral and dorsal weapons platforms. A top mounted beam, cannot fire at a target under the ship and vice versa. STO does NOT use that model, hence the slightly funky weapon hard points.
If STO were to follow the "REAL" ship models, your weapon slots would almost double, as would the associated cost of filling them, as you would need to have VENTRAL and DORSAL weapons.
Also keep in mind, your weapons choices would be more limited. The only vessel I've EVER seen in any trek show or movie with a turret, was a BoP. Also with the exception of the Defiant, and maybe one or two other very specific examples, I've never seen a Starfleet Cruiser with cannons.
Personally, I am willing to put up with a little bit of slightly skewed "hard points" to be able to use one weapons platform for both top and bottom and reduce weapon slots and COSTS. Since I wouldn't need to buy double the weapons.
Sometimes I think people get a little too carried away in their quest for "realism".
More firing point does not always equal more firepower.
More weapons arrays give the ships more coverage, but they still all draw from the same power source, which is why we rarely see starships fire more than one or two beams at a time.
I think its also worth a note that some ships in the shows do have phaser arrays in odd places such as on the nacels, under the main deflector, e.g. the intrepid class, and arrays by design are supposed to give the ship the ability to fire at multiple angles with a concentrated beam, which is why you see the array charge a point and the beam is then emmited, however BFAW is a little far fetched as each shot would be just a fraction of its full power. As has been stated before, the vast majority of ships in STO were never designed to use the weapons people want to give them so there hardpoints are by default not nessercerraly match up, and the ships in the shows had a tendency to chop and change what they were shooting with at the drop of a writers hat. At the end of the day its a game, the fact that they have managed to get the hard points to work as they do is commendable, I know full well how difficult such game coding can be after trying to help a freind do some of even the most simple coding for a game, also there has to be some gameplay adjustments to allow certain weapons to be able to fire where they need to on some ships for exaple the scimitar, if you want beams on it 360, then if they all shot from the main hull you would end up blowing through your wings when engaging certain targets, so to stop this they emit from some odd places, yet they still do what they say on the tin.
Jorhana Kreig: KDF, Tal'is: Romulan Fed, Tal'ar: Romulan Fed, Johan Paul Kreig: Fed