Because they come out looking almost exactly as they went in!
Some look identical (sweetcorn), some have slightly changed in colour (peas) or size (carrots) or texture (spinach) but all are very clearly recognisable. It doesn't look like they have been digested at all, so how am I gaining any nutritional benefit?
I am on a waiting list to see a gastroenterologist, so perhaps there is a medical reason for this.