UPFs tend to contain a lot of sugar (or sweeteners) and salt to make them more palatable, and to make up for using lots of what can be bland or unpleasant base ingredients. So cutting them out tends to lead to reducing sugar and salt consumption even though some non UPF foods can have high salt and sugar levels. Making home made bread allows you to reduce salt levels a little (though don't leave it out, bleurgh!). I sometimes need to consciously add salt to make sure I get enough, particularly if I'm sweating a lot in the summer.
But it's not just salt and sugar that cause health problems. There seem to be a lot of different potential mechanisms being proposed as reasons for the harms that UPF appear to cause and there's lots more research that needs to be done to pin this down. This is made more complicated by the fact that it seems to be a cocktail of different things happening in different types of UPF so it's hard to pull apart what is actually happening.
That said, one consistent theme is that UPF tend to be low in fibre, making them bad for our digestion and gut microbiome and meaning any sugars in them are absorbed very quickly (this is why eating an orange is better for you than drinking its juice). Low fibre foods are more calorie dense and take less chewing so it's easier to have more; you'd really struggle to eat eight oranges at a sitting and it would take a while, but you could drink the juice of 8 oranges in a glass in seconds.
The emulsifiers and preservatives that most UPF contains often thought to disrupt the gut (though again it's complicated, some emulsifiers and preservatives don't seem to cause problems and some even seem to have positive effects on gut health). Many artificial sweeteners seem to have an effect on our bodies' blood sugar responses, the sweet taste causing insulin levels to rise even though there isn't any glucose for it to take up, thus causing crashes and cravings. And common UPF ingredients like high-fructose corn syrup are bad for our bodies in so many ways.
And also UPF is designed to be hyperpalatable so people eat as much as possible, with manufacturers tweaking sugar and fat content to make them as appealing as possible. They choose which products to market based on which their testers eat more of.
It is perfectly possible that some types of UPF could be good for us (eaten in moderation), but we just don't know and it's increasingly clear that a lot of them are very bad for us, so with our current level of knowledge cutting back our consumption seems prudent.
@gogohmm tinned pulses are very convenient and minimally processed. I also use my instant pot a lot for pulses as they can be cooked unsoaked in 35 minutes at pressure (which is hands off time so just set up and go).