Why can't we replace a database with a cache entirely?

Question

Assume infinite budget. Why can a cache still not fully replace a database?

Discussion points

• Durability: caches are typically in-memory and volatile; databases persist data
• Consistency and transactions (ACID) that a cache does not guarantee
• Querying: rich indexing, joins, and ad-hoc queries vs simple key lookups
• Source of truth and recovery after failure