Like you said, you'd prefer to keep the keys you need close to the home row. So when a language has extra letters (compared to English), you preferably put those letters in the three letter rows, and carve them space as needed.
That's like saying "why do characters outside standard English exist?". I honestly don't understand what you're complaining about. People use international keyboard layouts because they want to write another language than English.
To hit return requires actually moving the right hand to reach return. How often is that tilda needed? That is a poor trade.
Moving the shift key away is even worse, because it's a modifier key. Now you cannot press shift with your left pinky finger while also reaching several of the left hand keys.
That is actually the original IBM PC XT layout from 1981: https://en.wikipedia.org/wiki/File:IBM_Model_F_XT.png
Then in 1984 USA moved to their own layout with the PC AT: https://en.wikipedia.org/wiki/File:IBM_Model_F_AT.png
But the rest of the world stayed with the original.
That tilda is needed every time I need to access my home directory on Linux, and the backslash key is also a pipe key when you press shift - so I can use a pipe in command line using a single hand.
I actually come from a country with US keyboard by-default. Now, whenever possible - I switch to UK layout even on MacBooks. UK layout is actually wonderful for development.
I can obviously see though why one can ask why is it this way.
Every single command you type ends with return. Unless you are typing long paragraphs in a word processor with line wrapping, you will be hitting return once every 80 characters or less on average (much less in the case of commands). Do you really need to reference a unix home directory once every 80 characters you type?
I do care about their placement, however - since that's where my fingers are physically limited.
Also, being used to it, I find a large Enter key more convenient than a wide one.
Unless you are doing something really "interesting", you should be hitting return with your right pinky finger. And assuming your other fingers are on the home row, your pinky finger has an easier reach to the US return key (on the left edge) than it would the international return key on the row above.
Now, if you suggested expanding the return key to exist where the { key does, then I could agree that it was an improvement. { is immediately accessible to a right pinky finger, and arguably { is less often needed than return - even in programming.
This doesn't make much sense for the UK keyboard layout, where I agree that the US layout is a bit better from a common use perspective. It makes more sense when you start looking at other variations of the international layout, though. Live in the Baltics? You're going to use the Nordic layout that has several of the most commonly-used letters with diacritical marks. The physical layout (number of keys, position of keys, etc.) are all still the same, so not only is it easier to manufacture but all a touch-typist needs to do is alter the software layout to be able to use any keyboard they happen to stumble upon.
speaking as someone who switched from DE_DE to US_INT_W_DEAD_KEYS it was to actually programming easier.
i did not need special german keys (äöüß) often while coding so having programming relevant `[]{}'\"|` at their expense, while still being able to access language specific symbols via dead keys, was preferable.
international layout also gives me the ability to type specific other-language symbols like `Øçµ` or special typography like `…‽¿` way easier compared to german
For people who need more modifiers and accents, my experience is less relevant.
Actually, that's how the return key originally was on the PC/XT model F keyboard, except that it was only 1 unit wide on both rows. It actually changed in the opposite direction with the PC/AT, and largely only on the U.S. variants if memory serves. The non-U.S. variants retain the PC/XT's original double-row return key, and make it wider (and make the keys next to it narrower, down to 1 unit).
Which keys are these in my international layout and how often do I use them?
I took the time to learn Dvorak (years ago), and I found it very much nicer for typing prose. However, I couldn't code with it (and I didn't try a code-focused version). Perhaps I should have tried different Dvorak variations to find one that was still better than QWERTY for writing while also being usable for coding.
I also tried a chorded keyboard (the BAT). With only a little practice I got to 60+ WPM (which isn't bad considering it's a one hand keyboard)... but it was just too laborious to code with since modifier keys and symbols took so much extra effort.
I'm still waiting for a neural interface... although if you give me a neural typing interface, then I'll want a neural visual interface so I'm not limited by monitors.
As an aside, why why why do people use the international keyboard layout? There is no possible way that it is more efficient given that two of the most used keys - left shift and (right) return are reduced in size and pushed further out of reach from home position. Worse yet, their lost space is taken by less frequently used keys. I cannot fathom why that design has remained popular outside the US.