A lot of web systems work this way - DNS records for example use a "TTL" which means "time to live." If the TTL is 60, then you throw it out of the cache after 60 seconds even if you have room in the cache, and you have no reason to believe it's invalid. This lets independent entities (like a DNS authority) make a change and get it rolled out everywhere.

I think the reason this is common is that proving cache invalidity is so hard, especially with the typical "dumb" cache appliances that are widely used. They just do stuff like cache the response bytes for a particular URL; they might not even understand HTTP beyond interpreting the request's headers, and certainly don't really understand the response.