Right. The FCC itself is not a law enforcement agency. Criminal prosecutions would be under the jurisdiction of the DOJ.
The DOJ, FBI, etc. all have limited resources. The allocation of $ is highly prioritized. To them, robocalls are way, way down on the list.
Note also, while the callers are not political spenders, the telecoms (which for spammers are secondary midsized ones) are big donors. So there’s a lot of political pressure to keep the money to the telecoms, and thence to Congress, rolling in. Many of the biggest telecoms that enable robocalls are located in more rural states. Which allows them to increase their leverage even more.
The US based robocallers are really good at spinning off company after company after company. So they will gladly shut down one company and promise not to do it again (saving the cost of the government doing a prosecution) and then just continue on with the next version of the company.
The warranty calls I get start with overseas call centers before transferring the call to the American thieves.