Because people buy after reading text and not after looking at images.
The best that an image on your site can do is to attract people to come: but unless you are a photographer or an artist who is actually selling images, then they will not lead to a purchasing decision.
People buy after reading the product description, or the specifications and technical details – and of course after seeing the price.
This is not to say that good images are not essential: a bad image can still stop a sale even if it cannot make a sale. How many times have you seen a product image on a site with a handy magnifying glass icon to make it bigger – and when you click on it, it opens an image that is much larger… – but consists of a massive white background with the product image exactly the same size as before.
What a total waste of time: and if you annoy a visitor to your site in this way enough times they are just going to click on the back button and leave, never to return.
In a similar way I have seen whole management teams spend the entirety of a website creation project stressing over the design of the site – the colours, navigation style and information architecture… – and then fail to give any consideration to the content, treating it as a commodity.
The best that the navigation on your site can do is to get people to the page where they can make a purchasing decision quickly and easily without annoying them. If the navigation on your site is obtuse and hard to use then people will just not use it.
When the navigation has got a visitor to the page with the buy now button on it then it has done it’s job: it is up to the text on the page now to make the sale. If there’s not enough information for the customer to decide whether or not to buy – then they won’t; if there’s no price – they won’t buy; if there’s no specification information – they won’t buy; if there’s no third party reviews and recommendations – they won’t buy.
It is the easiest thing in the world to create a site that doesn’t sell.
People buy online after reading text: if they don’t see the text they need to see then they will just return to the Google search results page and go to the next site down.