That's because relying on a TTL simplifies the concept of caching, and makes invalidation trivial, and also inflexible. It's used in DNS, which already was an

That's because relying on a TTL simplifies the concept of caching, and makes invalidation trivial, and also inflexible. It's used in DNS, which already was an example here. There is no way to be sure clients see an updated value before end of TTL. As a result, you have to use very conservative TTLs. It's very inefficient.