In the United States, there's no unified national unified university system, so there's no real answer to your question. Private universities do what they like (guided by custom). Public universities belong to state university systems and will be similar within one state.

But, yes, there is a very strong tradition in the United States that a four-year undergraduate education should include serious study of what is variously called "a core curriculum" or "the humanities" or "the liberal arts" or "Western civilization." And, while there a broad differences depending on arts vs. sciences you are not encouraged to "declare a major" or concentrate early on. 

As it happens, I went to the Massachusetts Institute of Technology in the 1960s and the curriculum was almost standard for everyone for the first two years. There was a little room for one "elective." But everyone was required to take four semesters of "Introduction to the Humanities," in which we read translations of Plato and St. Augustine and the Iliad and Shakespeare and so forth. And, being "a university polarized around science," everybody took four semesters of chemistry, physics, and mathematics. I just checked their website and nowadays "The Humanities, Arts, and Social Sciences (HASS) Requirement is an indispensable part of every student’s undergraduate education... each student must complete eight HASS subjects."

As to why, a common U.S. view is that the first four years of university, leading to the bachelor's degree, should be "education, not training." 

'I am studying Computer Science at the University of Pisa/ at Pisa' [assuming that Pisa is a well-known university].  If you say  ' . . in Pisa', the listener may think that you are attending another college in the city, or learning in some other way while you are living there.

The North American university syllabus is less specialised in the first two years than in Europe.  If you want to study in an English-speaking country, have you considered the UK ?  Fees [and living costs] in England are quite high, but many universities offer one-year Master's courses, and I don't think the standard is any lower than in North America.  It's also a lot easier in some other ways; as an EU national, you don't need a visa, you can take work if you wish, and stay in the country as long as you like.

As Overbyen has said...

I can't speak to Canada's university system but I believe that it's very similar to the US system.

In general, Univeristy in the US is broken down in the following ways:

Bachelors Degree - This is the first 4 years of university, generally the first two years are spent on general education with a focus on your general field of study and the last two years are focused primarily on your field of study. I think most first degrees in europe are more like the Masters Degree in the US.

Masters Degree - This takes 1 - 2 years and is focused purely on your area of study at an advanced level, often with a research focus

PhD - I think the doctoral degree is fairly universal so it should be the same everywhere.

To address your concern, yes the general studies classes can be a waste of time. My Art/History class titled "Paris" that I took during my first quarter at University certainly didn't help me with my career as a structural engineer in the same way that the Calculus, Differential Equations, Physics, etc did. With that said, the Bachelors degree in the US is supposed to be a well rounded degree that gives the student a broad exposure to many different ideas and thoughts. It might be the case that the generalized exposure to various areas of studies are well covered in you primary education. The US primary school system is probably less rigorous than most European school systems. 

Overall, I think Universities in the US are quite different from European Universities. In the US they tend to be large wealthy institutions with large campuses and operate in a world of their own. The largest Universities have endowments valued in the billions of dollars. I think that's quite rare in Europe with places like Cambridge and Oxford being exception. I could be mistaken about that though.

I think what you are talking about is general education, which are classes you have to take in addition to classes for your major. Pretty much every American university I know requires this, even Harvard and MIT. Not sure about Canadian universities, though. The point of gen ed classes is to give you more knowledge about the world outside of your field of study. It is somewhat like in high school, but at university, you actually have more freedom in choosing the class you want. For example, they say you have to take a history class, but you can still pick which history class you want (American history, East Asian history, European history, etc). It probably still doesn't sound too fun for someone who is a computer science major, but you can just choose a class with an easy teacher and get it over with.

When you get to the master's program, you will just concentrate on your field of study only and won't be required to take unrelated classes.