Exposure = Revenue (the business side)
The cognitive goods matrix:
- There are goods you are aware you need. This is your shopping list.
- There are goods you are unaware you need. This is the stuff you forgot to put on your shopping list.
- There are goods you are aware you want, but you don't exactly need them. Like a book you have on your wishlist, but you have 3 other books in your shelf to read anyhow, so no urgency ordering.
- There are good you are unaware you want, like anything on sale or chocolate truffles.
Exposure automatically promotes the unaware to aware status. Exposure can also lead you to buy things you don't necessary need ("Minimum order is £20, so I might just as well tuck this book I want in there").
Time on site = exposure
There is a maxim in commerce that goes: Exposure equals revenue. One way to increase exposure is to increase time out site. You don't want people buying from the search results page and checking out.
People spend more in a shop than they do online. This is simply because in a shop they are visually exposed to many more products than they are on a computer screen, and the physical nature of a shop means more browsing no search boxes. Financial advisors are unanimous about online shopping saving you money compared to a shop visit. Supermarkets constantly change the location of products so frequent visitors search harder, by that being exposed to more products.
How many times you entered a shop with a friend who was looking for something and ended up buying something yourself, despite this was never on the day's agenda?
Search result pages offer little opportunity for the business to expose you to more products - let it be related, those you previously searched for, or just random ones; when clicking on alternative products you are exposed to even more products. A product details page has much more space to do so, and users are likely to spend more time there.
The user side
I recommend reading chapter 2 in Designing the Search Experience, where a various search models are covered.
They all suggest that the search result page is not where people are likely to buy from. Then, there's the social effect - reviews are highly affective. Even if you are in an exact hit mode, you are more likely to use the previous orders page than the search feature.
So not having the button there is not exactly a usability tragedy from a user perspective.