I agree that many parts can be interchangeable but modularity can be done in a way that doesn't result in every car looking like the blobs we see about today. I wouldn't mind if there were only a handful of designs around with slight variations in trim and specs -so long as I like those designs of course! I know that is subjective so lets keep things interesting and focus on making things that get the job done but also in varying styles. Both can be achieved.
The need for fashion is made up, yes- but so is everything else. There's a balance between overly individualistic cars and the most cost efficient, reliable and uninspiring 4 door. Lately we have been leaning too far into the latter.
Alternators, starters, ac compressors and so much more could just be small, medium, large, extra large.
The price would drop dramatically.
Aaaaand, that's why they aren't modular. Take a look at something with obvious common parts like a side mirror. They are all intentionally designed adding special shapes to make them as unique as possible to the car. Never a simple square or rectangle.
Think about every GPU having 8GB VRAM. Nobody wants that.
Business don't want standards because they care about selling non-practical bits of the car for a price, like how good it can make you feel because you think it's cool, even if it's dumb (dummy exhausts, fake carbon, LED logos and fashion oriented design)
I wouldn't mind if there were only a handful of designs around with some tweaks depending on performance/comfort. Also, nowadays many different models share parts, like lights or rearview mirrors, showing there's a need for efficiency and scale bigger than the made-up need for fashionable cars.