A worker cooperative is just another business, albeit one where ownership is distributed among the workers. It still functions within the capitalist system and must follow its dictates. They start with capital, employ wage laborers, and must create profit to survive.
I mention this because really your question might as well be, "well why don't people just start more businesses to make more people employed?" The answer applies to co-ops as well: because unemployment is controlled by making it more costly to do business, forcing a need to cut costs (usually starting with labor) and often simply failed businesses. Co-ops are not free from this, they need capital to survive just like a capitalist-owned business.
The only way to free ourselves from this system is to expropriate control of the businesses and put it into workers' hands, which necessarily happens under the same conditions under which we could eliminate artificial unemployment (control of the state).
Put another way, we need to control the means of production, not be minor players still at the whims of capital.
Edit: I should probably mention that there's more to it than this but it requires getting into the weeds of Marxist theory.