The marketing budget being the same magnitude as the film budget is actually a large part of Hollywood Accounting. At least for the major studios, most of the "marketing budget" for a big film is in cross-promotional efforts with related entities that don't cost them actual money, or for tie-ins or licensing deals which their marketing partner is sharing some or all of the costs.

Also, movie theaters don't get half the box office gross. For big budget films, the distributor gets a declining share of ticket sales, starting with 90% for major films (Marvel, Fast and Furious, Mission Impossible, anything by James Cameron), and declining each week down to a low of about 10% after 2-3 months (depending on the specific contract between the distributor and the theater). This is a large part of why studios are now so focused on first-weekend gross: it's also their most profitable weekend.