It’s hard to believe it’s been 10 years since the last movie based on a “Saturday Night Live” sketch hit theaters. At least, it’s hard to believe until you’re about 15 minutes into MacGruber. And then you remember exactly why those movies stopped being made.

With very rare exceptions, the practice of stretching popular skits into full-length motion pictures has proved monotonously disastrous. For every hit like Wayne’s World, the genre has produced a string of flops like Coneheads, Stuart Saves His Family, It’s Pat, Superstar, The Ladies Man and A Night at the Roxbury.

Yet here we are again, inexplicably confounded to discover that something we find hilarious in 90-second snatches on TV could prove tiresome when fleshed out for the big screen. I don’t know why, but I had a sense that the creators of MacGruber were going to sidestep the curse; that they would have learned from the decades of mistakes and avoided them. Instead, they appear to have put those mistakes on a checklist and ticked off every one.

Will Forte stars as the dim-witted, highly distractible action hero with the flannel shirt, vest and blond shag. The running joke on the show, of course, is that every skit involves the same setup: MacGruber’s attempt to defuse a ticking bomb is interrupted by something decidedly less important, and his babbling is inevitably cut short by a fireball. It’s the perfect absurdist action-film spoof.

Absurdity is replaced by kitsch in the movie, as first-time director Jorma Taccone and his writing team apply their talents to a riff on ’80s action pictures. The military has coaxed our hero out of retirement to track down a ponytailed evildoer by the name of Dieter von Cunth (Val Kilmer), who’s acquired a black-market Russian missile and is intent on blowing Washington, D.C., to bits. The “h” in his name is silent, which should give you a pretty good idea of how low the filmmakers are willing to stoop for a laugh. And they stoop a lot.

MacGruber’s team is rounded out by the still-green Lt. Dixon Piper (Ryan Phillippe) and Vicki St. Elmo (a lost-looking Kristen Wiig), an old friend who composes pop ballads in her living-room studio when she’s not fighting crime in period pantsuits. They’re like the Mod Squad with brain damage.

The problem with the film, which was scripted by Taccone, Forte and “SNL” writer John Solomon, isn’t just that it directs its satire at obvious comic targets such as guns, cars and evil masterminds. Or that its humor is puerile and scatological. In the Age of Apatow, you expect puerile and scatological. You just don’t expect the naughtiness to feel so forced.

The fatal flaw is that this is the first movie based on an “SNL” sketch that neglects to incorporate the essential elements of that sketch — which are, after all, the only reason for the picture’s existence. Viewers don’t find the MacGruber bits funny week after week because they lampoon Stallone/Seagal-style action films or ’80s pop culture; they find them funny because MacGruber is clueless and invariably loses his race against the clock. So how do three gifted comedy writers — including the creator of the MacGruber character himself (Taccone) — manage to make a 90-minute big-screen version in which the hero never once blows up?

That’s like setting The Ladies Man in a monastery or spending A Night at the Roxbury without letting those goobers do their goofball dance, and it is also the reason MacGruber proves an unexpected dud.