Why do games tend to limit their form?

Why is it that games tend to rarely vary their form? You start with a set of activities-- running, jumping, shooting-- and while they may deepen and expand, they almost never change significantly through the entirety of the experience. By (very flawed) contrast, consider how much more malleable as a medium books and movies can be: They can switch genres, alter perspective and even change subject matter entirely. A movie like Good Morning, Vietnam, for example, starts out as a comedy but ranges into romance, drama, suspenseful action and even tragedy (Hancock, From Dusk Til Dawn, Vanilla Sky, District 9 and Click are movies that could also fit this example). What is it about games that so limit their form? Is it the maturity of the medium? The strict genre expectations of the audience? Or is it possible that one of the medium's greatest strengths-- interactivity and the process of engaging with it, which is basically learning-- is simultaneously a weakness of sorts? I lean toward this. Maybe the process of learning and mastering mechanics sets a kind of upper limit to what a game can depict and the sum of what experiences it can convey. Card games and board games seem to share this limit-- chess does not morph into poker, for instance, and it would be hard to see it do so (I wouldn't count playing both at the same time). Or maybe the whole question is flawed and the comparison to books and movies essentially apples to oranges. You might argue the bulk of movies & books fall into well defined genre categories, for instance, varying similar plotlines maybe much like a shooter or platform jumper or racing game varies levels. But I can't shake the idea of how most games can't really even switch genres let alone their overall form. When you start most, you know you'll be doing basically at the end what you started at the beginning, just maybe with different permutations and contexts. Imagine the howls of going from a shooter to a match-3 game, or from an RTS to a racing game, even if it was a smooth transition. Games that have attempted this, like Spore (or the little known Gordon Alliance decades before it), often run afoul of the problem of sacrificing depth for breadth or suffering from mechanics that just don't cohere well together. What do you think?