Blame OPEC, taxes, weak currency, rogue environmentalists, increased global demand, and speculators.
Oil companies themselves don't make the huge fortunes most think they do unless a well is an unusually high producer. Their margins in general are pretty slim. Many a would-be Jock Ewing soon end up doing something else and perhaps even bankrupt after a short-lived venture into wildcatting.
It takes a very large investment to drill on land, and even more to drill in water. And wells don't always produce.
And if they do, the oil has to be transported and refined, both of which also cost money and use fuel themselves. Politicians and environmentalists who extort from oil companies also need to be greased so that operations are allowed to start up and continue. And much-needed workers paid very good money to be motivated to do the work.
A an artificially weak dollar (printing excess money, keeping rates artificially low, substantial Fed Reserve asset purchases, etc) can be a good thing in controlled amounts in fragile economic times when growth needs to be stimulated. But it cannot go on forever, and will have undesirable consequences if overdone.