No it's not "built on it." Left wingers always say stuff like this, they find some negative, illegal or undesirable behavior and then claim capitalism is "built on" the despicable thing. It's built on people trading freely with one another to make themselves better off. Now that's not to say a restricted supply isn't desirable for the entity that deals in it, it's completely natural that some sellers of something would seek to rig the system to restrict it for their own profit, but that is where the government is supposed to step in and forbid such anti-competitive behavior. It's why we have anti-trust laws, and it's why any economist, left wing or right wing, will acknowledge that collusion to cut off supply and undermine competitive bidding in the market must be regulated and stopped by government. That is a legitimate governmental function.Capitalism is built on artificial constrictions of supply.
But instead of government stepping in and saying "this is a cartel, and is anticompetitive, and it's forbidden by our laws," you compete in the market like everyone else, we don't tolerate market-hindering collusion and cartel schemes, instead of doing that, the government officially sanctions these cartels and enables them. Some in legislative positions think this practice should end, but they face major battles getting that through because unions spend an insane amount of money installing political leaders to protect their cartel. It is corrupt to the bone, but they work hard to keep their cause noble-sounding.
Negative behaviors that screw others over are natural, predictable flaws in humans. It's not unique to capitalism. It's a human characteristic. People don't suddenly all become perfect beings under socialist or communist regimes. People want to make themselves better off and will in some cases resort to schemes that are unfair, harmful, and are therefore made illegal. Cartels should all be illegal. So should monopolies. Labor cartels (unions) are a special exception that are not illegal, but they should be, or at least the anti-competitive tactics they use to protect their members and screw over outsiders.This is not something unions have invented or are the only ones to utilize, it's the fundamental aspect of capitalism.