lowkey bringing back letters of marque for the [insert current conflict] is a perennial topic

Can someone help me understand why this wouldn’t be desirable? If the sanctions are ineffective, why not pursue more effective strategies with proper incentives?

Because they don't want to. There might be a couple good reasons, and there are probably a lot of bad reasons. On one hand it's a little too close to actual war. On the other hand, people in politics and people with lots of money tend to have strong affinities for one another.