Most any kind of processed food has loads of sodium in it, especially the instant-type meals you're talking about. Sodium acts as not only a flavor enhancer but also a preservative. So to make food that has had all the natural flavor processed out of it palatable and extend shelf life, it needs ridiculous amounts of salt. This is where Americans get most the massive excess of sodium in their diets that you hear about.
Now food that is more-or-less home cooked is going to have a far lower sodium content because it's only going to have the salt that you add to it. Using your example of spaghetti: the noodles will have a pretty low sodium content and the jar of sauce you buy will be higher, depending of what type , but probably not in the range of an instant meal (unless you get a jarred alfredo, that shit is crazy salty). The jarred sauce is obviously processed but much less so than a Lean Cuisine. If you were to make your own sauce from canned tomatoes, onions, garlic, herbs, ect., you'd not only have a far superior tasting meal, but also a much healthier one. I'm not going to go on a rant about how great learning to cook is - you get the point.
On sodium and health: If you're a basically healthy person eating large amounts of sodium isn't necessarily the worst thing in the world for you. Your kidneys do a fine job of regulating sodium and fluid content in your body and you should be able to deal with salty meals just fine. OTOH if you have issues with your health, namely high blood pressure, kidney problems, excess weight, etc., you'd be much better off without the high sodium food. I'd also recommend a lower salt intake so your palate isn't blown out by salt. If you're eating high sodium foods all the time, "real" food isn't going to taste salty enough for you. Once your palate is adjusted away from the processed stuff, you'll find that it really does taste like shit and you'll wonder how you choked it down!