Just because a contract proclaims that a content creator is doing "work [made] for hire" does not make it so. A work made for hire may only be:
a work specially ordered or commissioned for use as a contribution to a collective work, as a part of a motion picture or other audiovisual work, as a translation, as a supplementary work, as a compilation, as an instructional text, as a test, as answer material for a test, or as an atlas, if the parties expressly agree in a written instrument signed by them that the work shall be considered a work made for hire. For the purpose of the foregoing sentence, a "supplementary work" is a work prepared for publication as a secondary adjunct to a work by another author for the purpose of introducing, concluding, illustrating, explaining, revising, commenting upon, or assisting in the use of the other work, such as forewords, afterwords, pictorial illustrations, maps, charts, tables, editorial notes, musical arrangements, answer material for tests, bibliographies, appendixes, and indexes, and an "instructional text" is a literary, pictorial, or graphic work prepared for publication and with the purpose of use in systematic instructional activities.
17 U.S.C. § 101 (definition clause 2) or a work prepared by a statutory employee within the scope of that employee's duties.
The key problem — especially for freelancers — is figuring out whether the final product is one of the nine eligible classes of work. If it is not, then the result is not WFH. It's pretty easy to exclude most of those classes in most instances; for example, only "contribution to a collective work" and "compilation" seem even remotely possible for a game (as interactive material is otherwise excluded from "motion picture or other audiovisual work"). However, "collective work" and "compilation" both refer to products in which the individual contributions remain discrete, identifiable elements, the classic examples from print publishing being a periodical and an anthology. The contract cannot enforceably turn ineligible material into WFH any more than it can enforceably declare that a postcard is the ceiling of the Sistine Chapel.
That doesn't stop the gaming industry (among others) from including WFH clauses in contracts for freelancers to edit rules, or further develop game concepts, or create integrated backstories and settings, or other common freelance game elements. Further, it wouldn't prevent them from doing things correctly: Requiring a transfer of copyright. But tradition — specifically, a misbegotten tradition of relying upon audiovisual-work contracts (often themselves out of date, having been developed as "customary" under the 1909 Act) as templates — has made improper claims of WFH customary.