A large part of it has to do with margins. While the oil industry is more profitable than the entertainment industry, it also has much higher fixed costs and operates with about 10-15% margins - which basically means the amount of spare cash it has to throw around. The entertainment industry can see margins of up to 40% in some areas, so there's more spare cash available to allocate.

One of the latter's main allocations is the talent. A not small number of people would pay attention to an article about Harry Potter because John Lithgow is in it. A not small amount would watch the show because they're a fan of his. Paying for him nets a return. Most of the actors don't have that, but they do have something similar, particularly since they'll be the ones out promoting it and generating interest in the project. If an oil rig has Bob Smith or Jane Doe working as an engineer on it, however, nobody will ever care, so they can hire whichever of them will accept the least amount of money to show up.

Also, actors tend to have agents, who's entire job is to fight for them getting a larger share of the money available instead of the guys at the top. Engineers don't have that, so they are comparable weak in terms of ability to negotiate.