I have found myself in a similar predicament where I only train what I see is growing and do not do anything else. However, I found out over the years that the muscles that you do not see growing are actually the ones you need to focus on. Here are some reasons why you need to train your back.
Having a weak back is similar to a chipped pillar that supports the second floor of your house. Imagine that this is the only pillar that holds up your entire second floor and due to the fact that you sleep on the second floor, you started bringing heavier furniture up the stairs so that you have more access to it. Sooner or later, the pillar will simply collapse under the shepherd pressure that you put it through. It is the same for your back. The point I am bringing across is that your back supports all the muscles you would like to grow bigger on the top half of your body. If you find that your back is having a hard time coping with the sheer weight you put it through, it is a warning sign for you to start your training back. Imagine that your dead-lift stalls as 20kg and your upper body itself weights 30kg. Does it seem ridiculous now that you are stalling and not finding anything muscle growth on the top half of your body?
The next reason why you should train your back is because everything in life is dynamic. It is not so isolated compared to how your gym training might be. Ask yourself this, do you lift your couch with one hand just like Bam Bam from the Flintstones? I guess you know the point I'm driving at right now. Everything that requires movement requires a positive effort from the base down so that you can generate the least amount of force from the upper body which is being supported by so little muscles compared to the bottom half of your body. You will also find that if you train your back, lifting or shifting of heavy things like your furniture is a breeze because you can generate more force from the base of your body, then making your life easier.
Lastly, the reason to train your back is so that you can get nicer looking abdominal. I bet you did not hear this before, have you? The reason why this is true is because when your core is well supported and burning calories off easily, it is easier to get a six pack abdominal structure. However, you would also want your six-pack abdominal to look nice and even. With a well trained back, your body is always in equilibrium and it is not biased to one side, so your muscles will grow more evenly and be dependent on both sides rather than being dependent on a single side.