You can't paint only some things in lead on a whole economy scale for similar reasons to Amazon not wanting to enforce separated items in warehouses.
As long as "approved for tanks" lead paint is in the supply chain, who exactly keeps the use only to tanks. There are sufficiently durable alternatives that the lowest cost is to outright ban lead paint instead of setting up an enforcement framework to chase lead paint abuse while also paying added costs for after effects of easier cheating.
When I was a teenager, we painted our barn with lead paint. Let's just say that it didn't come from the local Sherwin-Williams. If it's for sale anywhere, the gray/black market will take care of the rest.
As long as "approved for tanks" lead paint is in the supply chain, who exactly keeps the use only to tanks. There are sufficiently durable alternatives that the lowest cost is to outright ban lead paint instead of setting up an enforcement framework to chase lead paint abuse while also paying added costs for after effects of easier cheating.