Spending time trying to fund and manage black market networks, raiding recycling centers, buying and dismantling washing machines for chips etc. is all time that the Russians aren't putting towards streamlining cruise missile production.
At the same time sanctions and export restrictions impose costs on the US and its allies. Those costs aren't worth it if the restrictions aren't themselves effective, which they almost certainly won't be for common off the shelf components.
You have to remember that both investigation and prosecution involve a large amount of discretion, so the cost of the sanctions regime is dynamic and variable. Enforcement can put a small effort against low-value components and put a large effort into violations of high-value components. If the small effort put against low-value components deters some activity, then the goals of the sanctions regime are furthered.