Saying a fireball could possibly ignite worn or attended items is only applying many people's personal knowledge and experience to an effect (fire damage). Granted there I many things in. this world beyond my personal experiences, but I am sceptical about fires being known to allow anyone to read someones mind, or to ties shoes, or do the laundry. Now in-game we are talking about magical fire. Maybe someone wants to include such in theior game world. Why, back in the real world old days, we had a cleaning product that cleaned like a white tornado.
As for Mr. Crawford's tweeted ruling (not quite the same as a rule), did he specifically say that it was against the rules for Fireball to ignite an attended item, that the DM would be breaking the rules? It is more work for the DM to keep track of extra saving throws, and what the effect of creature saving throw success or failure might have on the attended object's saving throw. Leaving it out of the spell description means the DM doesn't have to do more work if he doesn't want to. But the RAW leave many things to the DM's discretion (rulings, not rules). I don't consider it a house rule (rather a DM ruling) if sometimes worn or attended items ignite, or even which unattended items.