Why is everyone surprised at the idea of "making vegetables"? Have you not heard the word "make" used to mean "prepare"?
The verb make is used especially in the phrase make the beds and
when you are talking about preparing or cooking food:
He makes a great lasagne.
I’ll make breakfast while you’re having a shower.
You can also say get, get ready and, especially in North American English, fix for preparing meals:
Can you get dinner while I put the kids to bed?
Sit down—I’ll fix supper for you.
Source: Oxford Learners' Dictionaries
I wouldn't use 'make' in this context. I'd make a specific vegetable dish - cauliflower cheese, or mashed potatoes, say. If I were just boiling or roasting the vegetables, I'd use those verbs, or 'cooking'. Everything I did beforehand would be 'preparing'. I might 'make vegetables' if I were sculpting them out of clay or wax. I wouldn't say it's wrong to use 'making' in the context of making dinner, but it sounds unnatural
That's your prerogative not to use the word that way, but it does not change the fact that it can be used that way and is indeed used that way. Which is what makes language beautiful: a word can have more than one sense and it really depends on how flexible one wants to be with it.
I know a lady whose mastery of English, her native language, is pretty impressive. Yet to her, saying you felt lousy made no sense because as far as she was concerned, something lousy is infested with lice. LOL
So it is not unusual for native speakers to have some rigid rules about words they have gotten accustomed to using in only one sense and to find any other use odd.
LOL On a serious note, the word "make" has a lot of definitions. One of them being "to prepare or put in a condition that is fit for use" which to me is what happens when you chop up veggies to make a salad or you cook them.
If you insist that "make" only means "to bring into existence" then pray do tell, every morning when you make your bed, exactly how do you do it? Do you get wood and nails and a saw and hammer and engage in carpentry to "make your bed"? Of course not. Why not? Because "make" doesn't just mean "build".
Collins Dictionary lists "prepare" as a synonym for "make" in the sense of "cook".
I can actually imagine this exchange:
I have made dinner. Are you hungry?
I'm famished. What did you make?