Muta can lose because he has lost to less talented guys then Rock and Angle before. He lost to guys like Arn Anderson and Barry Windham, who are great but not on the same level as either the Rock or Angle. If you back to Starrcade 1989 Muta lost three straight matches to Ric Flair, Sting, and Lex Luger. Those guys with the exception of Luger are more on par with Rock and Angle, but Muta lost to all three of them. Muta is far from invincible.
When was Muta's prime? I think most would agree that it was between 1993 and 1999. So, that would throw out losses in 1989 and 1990 right there (so, goodbye losses to Arn Anderson, Ric Flair, and Lex Luger; his loss to Luger was only a DQ, by the way). Of course, you can choose to include these matches if you want, but then we'll just bring out The Rock's earlier career. I can't really say anything about Angle, as he was booked as a legitimate threat to anyone from the get-go. But, he has had some pretty embarrassing losses as well, and he doesn't even have the excuse that these were at the beginning of his career; both The Rock and Muta do have this excuse.
So, that leaves you with a loss to Barry Windham, a one-on-one match where it basically took Windham, an ox of a man, ten minutes of brutal suplexes and slams to finally put Muta away for a three-count that Muta sprung back up from as soon as it was over. Should we look at the differences between this type of match and the one Muta is in right now? Let's.
One, this is a no-disqualification match, in a cage. I can't even imagine the kind of havoc Muta would be able to wreak on his opponents in this type of match, given that so many foreign objects would be available to him and that he wouldn't even have to be cautious of using them. One might say that this limits Muta's evasion tactics, but this assumes that Muta's opponent would be able to concentrate on Muta at all times, which leads me to my second point. Two, this a triple-threat hell in a cell match, and the overwhelming outcome of such a match is that there is no strategy (that is to say, none of the two men decide to cooperate and take out their mutual threat first before going after each other). So, this essentially gives Muta a free ticket to hang around and only interfere with the fight between Angle and Rock when it's necessary to break up a pin or submission attempt.
What strategies do Angle and Rock have? Do they have experience with someone like Muta? I would say no, because there has never really been a wrestler like Muta in WWE; that is to say, a wrestler who relies much more on his smarts and psyching out his opponent than on seeing if he has the athleticism necessary to do nothing more than physically outdo his opponent.
Keep the objections a comin', please.