Hops have been highly prevalent in beer brewing since the early middle ages. They serve multiple purposes: adding to the taste of the beer, promoting yeast growth over bacteria, and acting as a preservative.
There are substitutes for the use of hops, collectively known as gruit. There are also several varieties of unhopped beer. As of yet no large scale brewing company has introduced beer made using either technique. Instead within the hops industry substitutes lie amongst different varieties of hops. Centuries of breeding have produced around 80 varieties of hops used in commercial production today.
Handbook of Brewing excerpt (Google Books)
The above table shows that hop production is just as oligopolistic as beer production. This leaves room for profit margins on both sides. Also, many hop varieties are owned by just one firm, which adds to the bargaining power held by hop companies. Beer firms counter this by owning enough hop farms to partially ensure their supply and by using different mixes of hop varieties, especially in beers that are not very “hoppy” like Bud Light.