It's no secret that fame and talent have never been synonymous in Hollywood: exceptional actors will spend years slogging away thanklessly in indie movies nobody watches while attractive young performers effortlessly win A-list roles they clearly don't deserve. It's just the nature of the industry.
However, even a bad or mediocre actor usually stumbles their way into a good performance sooner or later, when the material is right or they're teamed with a filmmaker who knows how to use them appropriately.
For instance, Vin Diesel gave his single decent turn in Sidney Lumet's 2006 dramedy Find Me Guilty (though nobody saw it), Hayden Christensen was terrific in Shattered Glass, and Jean-Claude Van Damme killed it in JCVD. And that's basically their lot.
As for these 8 actors, however, not once have they turned in a genuinely good performance. Despite most of them headlining many major movies over the years, they've all been unremarkable at their best and really quite awful at their worst.
It's actually not that easy to give consistently forgettable and bad performances in films, because the law of probability dictates even the worst actors will eventually be utilised effectively, but sadly not in these instances.