Yes, you are missing something here.
You are missing the fact that if you are using dive tables rather than a computer, you don't just use a dive table. You have to have some way to measure depth and time, and both of them can fail as easily as a computer. In fact, the only instrument I ever had fail me under water was a dive watch, and fortunately that watch failed me in a swimming pool. Most importantly, it failed me by stopping for a while than going again, so I had no idea it had failed. I was teaching a class at the time, and I was using it to judge when I had to get the students back to the surface to begin the next session of the class. When I got to the surface, I looked at the wall clock and realized my watch had somehow lost 10 minutes while I had the class in the deep end of the pool. If I had been using a computer instead, the screen would have gone blank on a failure, and i would have known something was wrong. Using the watch, the only way I knew something was wrong was when I saw the wall clock.
If I had been using that watch with tables during a dive, i would have spent 10 more minutes at depth without realizing it, and I probably wold have gotten DCS.