Here's a simple reason why Amazon is so obsessed with growing its fashion apparel business

In a recent job posting for a software engineer, Amazon wrote that it wants to become "the number one online shopping destination for Fashion customers."

That, combined with the fact that Amazon has quietly launched 7 new fashion brands recently, only deepened speculation that the world's largest online retailer is serious about expanding its apparel business big time.

But why would Amazon be so obsessed with the fashion apparel market, an area it owns less than 5% market share?

Here's the simple reason: Clothing has the highest online penetration rate among all retail categories in the US, and Amazon already leads the online apparel shopping space.

According to a note published by Morgan Stanley on Monday, 52% of US shoppers have bought clothing online in the past 12 months, by far the highest rate among all online retail categories:

And within the online apparel market, Amazon holds a distinct lead compared to other retailers that offer online services, far outpacing big box retailers like Walmart, Macy's, and Target. In fact, 45% of US shoppers indicated they buy their clothing on Amazon, up 6 percentage points from the same quarter of the previous year, the highest growth margin among the 10 retailers surveyed:

On top of that, online shoppers are expected to spend even more on clothing this year, which will only help Amazon capture a larger share of the combined $1 trillion clothing/grocery market in the US, Morgan Stanley noted.

Disclosure: Jeff Bezos is an investor in Business Insider through hispersonal investment company Bezos Expeditions.