why continue to make the planes when you have no idea when you can actually push them out the door? given that they continued to produce the MAX while they were grounded, they must be running out of storage space...
Because they have a huge backlog of orders to fulfill when it does get re-approved and it's very expensive to shut down and idle the production process (besides Boeing themselves there are 100s of suppliers)?
By stopping production, some of their suppliers will go out of business. It was a gamble to keep producing planes without knowing when they’d return to service, hedged against the cost of rebuilding parts of the supply chain from scratch.