Because the distinction is whether one's primary activity is using capital to gain income, versus working for pay. "Ah but a worker may own some shares!" isn't important. It's tough to nail down color names at transition points, too, but you add a tiny bit of white to some very red paint and no-one's gonna start calling it pink. It's still red.
[EDIT] at the heart of the misunderstanding, here, is a false syllogism, I think. "Pink paint is red mixed with white, you mixed some white in this red, therefore it is now pink"; "The capitalist class own capital, this worker owns some capital, therefore this worker is part of the capitalist class"; "The capitalist class owns capital, this guy owns and operates a hot dog stand, a hotdog stand is capital, therefore this guy is part of the capitalist class". It's quite similar to another that's often seen: "Mugging is taking something by threat of force, governments ultimately back taxation with the threat of force, therefore taxation is the same as mugging" or variations that end up at at some form of "taxation is the same thing as theft, so your thinking is inconsistent if you don't transfer all your bad feelings about theft to taxation, as well".