They currently have two similar blocks — "Customers who viewed this also viewed" and "Customers who bought this also bought" — and I assume they learned through

They currently have two similar blocks — "Customers who viewed this also viewed" and "Customers who bought this also bought" — and I assume they learned through massive analysis that these two blocks lead to more sales than the "people who viewed X ultimately bought Y" block. Whether having "people who viewed X ultimately bought Y" is more useful to you is, probably, a matter of personal taste, although one could say that leading a customer to a product they feel like buying is a good proxy for "helpful" and in that case Amazon has obviously determined empirically that "people who viewed X ultimately bought Y" was not as helpful as what they've replaced it with.