Part of it is arrogant bullshit stemming from a complete inability to accurately remember what the experience of being young was like. Most people suck ass at honest self-assessment at every stage of life, so they fall into the trap of thinking they and their cohort were different than "kids these days" and are unable to empathize with or relate to young people because of this.

Part of it is that kids genuinely just aren't fully developed neurologically, in ways that frequently lead to making shortsighted decisions with negative results. Unfortunately that's still true way past 18, it's an arbitrary threshold and the expectation that people should instantly stop acting childish at 18 is just as stupid as assuming that anyone younger than 18 is categorically incapable of making the right decisions for themselves.

The specific examples you're using have some reasonable claim to option 2 depending on the context. Even as adults most people struggle to some degree with eating healthy nutritionally balanced diets, and children generally have less self-discipline so it can be more of a problem for them. Forcing kids to eat things they don't want is a bad idea though, you'll just give them a complex about eating and make them hate that specific food forever. My parents rule was that I never had to finish anything I didn't like, but I had to try at least one bite of everything before deciding I didn't like it. Worked like a charm, I eat everything.

Screen time is similar, plenty of adults struggle with regulating their gaming and social media usage, and the companies that make games and websites get paid more when people use their stuff more, so they have an incentive to design things to be addictive and predatory. Adults assuming that all screen time is bad for kids are stupid, but it is possible that an adult might recognize a predatory system that a child wouldn't have any experience with and save them from being taken advantage of. Best practice is to actually pay attention to what's on your kids' screen and discuss it openly with them. Unfortunately a lot of adults don't want to do that because it's difficult, they default to either assuming all screen time is bad or to not trying at all, so their kids are either needlessly restricted in their growth or exposed to the worst the internet has to offer. Both bad.

Tldr the adults you're dealing with aren't necessarily completely wrong but they're probably still full of shit