Why would you assume situation is worse in India by the way?
Maybe. I suspect many garment are not manufactured to really tight tolerances. Once I found a perfectly fitting trouser at a brick-and-mortar shop. It was so good that I thought I will buy two of the same size right away. Turns out the second was much larger than the first even though according to the label they should have been the same.
So yes, it shouldn't be much extra effort to measure one of each size of each new model. But if you have to measure every single garment that can become a nightmare. And I don't know how much would it cost to tighten up the tolerances in their manufacturing.
Besides I wouldn't know how to measure my own back precisely. Probably a tailor would know a standardised way to measure it but that is not something you can assume your costumers will know. And if the shop is measuring the backs of their jackets differently than the costumers do they will still get returns. Of course they can choose to not accept them, but who would buy from such a shop?
I suspect the platforms have decided that it's not worth the trouble to wrangle a large number of sellers into maintaining accurate and consistent product listings.
For whatever reason, people just assume "I'm a S" or "I'm a L" and borderline cases search reviews for "it runs small" or "it runs large". Women's sizes are sometimes numbered (because, of course, women only vary on a single dimension).
Is it actually fraud if it's a 'no questions asked returns' policy, and does not specify anywhere you aren't allow to wear it for an extended period of time before returning?
Scenario: You buy a laptop from a local shop, after returning it in person, you say to the Owner: 'Thanks for the laptop, I just needed it for my international trip.' What exactly can he charge you with legally?
EDIT: Ah okay, upon further research it seems like "fraud" and "unlawful" aren't the same here.