Mono fruits is convenient but not ideal. Our taste buds get tired of it quickly and we lose our appetite quickly. Eating just fruits is already difficult to get enough calorie. Now, mono fruits making even more difficult to eat.
Banana by itself is too sweet. Lemon by itself is too sour. By when you mixing both. You get the perfect combo taste. Because it taste better, we can enjoy more and eat more to get enough calorie.
Every fruit have it's strength and weakness. By pairing the right kind of fruits together, we get 10 times of excitement. And hopefully getting enough calorie.
There is no evidence that our body can only process one kind of food at a time.
Naturally, it's also possible to find banana and lemon at the same time. And when you are eating, just put them together.
What you explain doesn't fit my own experience. I prefer eating the fruit by itself (if it is good quality)
I've been at periods eating half of my calories from only mangoes and I loved it! It didn't get boring at all.
However, if the quality of fruit I have available isn't good, I prefer mixing - preferrable in smoothies.
It can be challenging to get enough calories if you haven't been eating only fruits for a few months. It took my about two months before my stomach had gotten used to the high water-content fruit and gotten flexible enough to meet my caloric needs. Smoothies help, and bananas and fatty fruits and dates.
I'm not saying there's evidence that our body can only process on food at a time, I'm just saying that if we eat one food at a time it is processed more effortlessly, I'm not going to build up a scientific statement for that, rather I'm just speaking from my own experience from experimenting with it. The more mono-meals I eat, the better digestion I have.
It, took my a long time before I started digging mono-meals, and as you say it can be considered boring. Nevertheless I think this is mostly because we are so used to stimulative foods, and that it takes time to be able to appriciate eating simpler. I'll come and give you some amazing mangoes one day, and I'm sure if you try them you don't want to eat anything else for that meal, I think it's all about the quality, it has to be super-good. But we're all so different too, so maybe that's just your preference; to not eat mono-meals.
It's super and beautiful we all think differently. Imagine you go to a garden and all the flowers are one kind and one color. As long we don't kill innocent animal like an extreme mad man.