Because putting a ship in the hands of an AI that will inevitably decide to use those relativistic weapons to DESTROY ALL HUMANS is really not clever?
You don't need some kind of sophisticated AGI, just a regular computer. Heck, even if an organic creature was on the ship it would not be capable of doing anything except telling the computer it was okay to fire the weapons or engines. Why not do that remotely?
Something which you can reduce the size of by using a lighter spacecraft. Bigger spacecraft = more mass = more energy required to move the mass = bigger engine = more fuel consumed. A spacecraft a meter across with a proportionally sized fuel tank will be just as capable of maneuvering as a spacecraft kilometers long with a kilometer long fuel tank.
Kinetic point-defence? the sort of thing that would be really useful when your enemy's firing homing missiles at you?
Think of the distances involved in space. Well actually.. don't, because it's not possible for the human mind to conceive of those distances. Now, let's say you have a gun which can fire a bullet at 20% of the speed of light, and bear in mind that's a really powerful gun! It's going to be firing with the energy of several nuclear bombs. But yeah, assume you have it.
Firstly, newton's third law. Firing that gun is also going to create an equal and opposite reaction in your spacecraft, and when you're using that amount of energy you're talking a lot of recoil. Frankly, that gun would make a really good engine, just a really poor weapon because secondly, whatever you're trying to hit is moving. As mentioned, anything engaging in space combat is going to need to be constantly changing speed and moving as erratically as possible. Let's say you're shooting at something 0.2 light seconds away, which is not very far in space terms, it's about 1/6th the distance to the moon. Well, firstly you can't actually see where the object you're firing at is, only where it was a fifth of a second ago. Secondly, it's going to take a whole second for your bullet to arrive. Given that the object you're shooting at is probably only a few inches long at most, the odds of successfully making contact are, quite literally, astronomical.
By contrast, a laser fires at the speed of light, which is still pretty crap because it takes .2 of a second to hit anything, which means you're trying to hit with .4 seconds of lag, but is vastly, vastly more likely to hit anything. Plus, you don't need to take worthless ammunition into space.
Good luck getting anywhere at relativistic speeds if the front of your ship is an engine instead of an ablation shield
Good luck getting anywhere at relativistic speeds at all. The only currently realistic plan for attaining relativistic speeds involves strapping most of earth's nuclear arsenal to a spaceship and detonating it one by one. Done correctly, that could accelerate a spaceship to perhaps 10% of the speed of light, assuming you didn't want to ever slow down.
Now, if anything did hit your spaceship at 10% of the speed of light, I'm afraid an ablation shield probably isn't going to save you.
Did you actually just describe a bullet-missile homing-rocket gamma-laser battle between two break-dancing supercomputers in space as boring?
Heh.. Yeah, poor choice of words. What I mean is it wouldn't be comprehensible to a human because it would be taking place too fast, at distances too great for a human to be able to observe or even conceptually grasp and success or failure would essentially come down to impossibly fast number crunching and a bit of good luck.