Because the pretrained chatbot is the flagship product of an AI company in 2025. They want to sell this product to customers who can't spell RLHF, never mind have the (substantial) resources to do their own training.

If Anthropic were getting into the vending machine business, or even selling a custom product to the vending machine industry, they'd start somewhere else. But because they need to sell a story of "we used Claude to replace XYZ business function", they started with Claude.