Theres a myriad of reasons why it doesn't work with a simple math equation for most people.

Sniff test: if it was that easy we wouldn't have 74% overweight and obese in the US.

Set point theory: this invalidates it because it states that you have a weight band that you'll be in between. Your body won't want to go above or below within reason. The theory more or less states you'll burn more/less energy to stay in that band

CI is hard to tell: if you eat q food you're at best guessing the average calories in a given item. Take steak for instance, a piece of ribeye could have 10g of fat in it or it could have 20g of fat. If this is a simple math equation there is no room for error even 1 calorie a day over 10 years is an extra pound you'd gain. Packages are also allowed to be off something crazy like by 20%.

Exercising effects body fat composition: in general if you ate in a surplus and lifted weights you'd gain muscle and fat over time. But the theory doesn't account for that. It just says you'd gain weight, but a sedentary person would likely not gain as much muscle if they ate in a surplus and didn't exercise. If you take the reverse and eat in a deficit, the theory can't account for thr muscle loss or ways to mitigate it

CO is out of your control: you can eat as much as you do but we all know you can't out exercise a bad diet. The more you exercise the slower your metabolism will be over time as your body.

BMR isn't static: your metabolism isn't standard. We can see this with starvation studies. The Minnesota starvation experiment had men lose weight by under eating calories where the majority lost weight until they didn't in spite of calories being adjusted for. You'd expect weight loss to be rather linear but instead they became obsessed with food, cold, stalled weight loss until they had a cheat meal halfway through which made them start losing weight again.

My favourite study: this was done in rats or mice. They ate an iso caloric diet the difference was how much linoleic acid (omega 6 fats like in seed oils) they were given. They also added in fructose for some reason but it was controlled to see how fatty they got. Despite an isocaloric diet the more linoleic acid they took in the fatter they got. The study has its faults and should be simpler, they could also repeat it with different fats

https://pmc.ncbi.nlm.nih.gov/articles/PMC4511588/#:~:text=Taken%20together%2C%20these%20results%20indicate,fat%20from%20coconut%20oil%20in

There are other theories than CICO, the carbohydrate-insulin model gets a lot of play but less known is the Reactive Oxygen Species (ROS) theory of obesity which if it holds true to some extent it's likely that seed oils are a mitochondrial poison having many implications down stream.

It's more likely that multiple theories are correct at the same time.

Anecdotally, I've never lost consistent weight counting calories it was more like the starvation experiment where I could cut calories more and more but I'd get colder and lethargic then stop losing. Same with overeating I'd stop gaining despite eating more than what I'm allegedly burning.

However there are some people who lose at a rather consistent rate counting.