Because we ALL need an "intelligent" smoke alarm and central heating, don't we? Yeah... no... the phrase "sledgehammer to crack a nut" JUMPS into my mind. The more technology there is, the more there is to go wrong. Seriously, a "connected" smoke alarm? Listen okay, a smoke alarm DETECTS SMOKE - that's ALL it needs to do. If you need it to do more, it's going to become progressively more unlikely that it will be able to RELIABLY (kinda important for a LIFE SAVING device) do the one, SIMPLE thing it needs to do, which is to tell you "HEY! I DETECT SMOKE - TAKE IMMEDIATE ACTION" - you're not going to be admiring how "cool" it is for being "intelligent", when your house is on fire.
I've been a qualified electronic engineer for 24 years, and an idiot for ZERO out of my 40 years. All these "intelligent" and "connected" gadgets just exist to validate the existence of smart phones, by dumb people.
We don't need things like the Nest, no. But it is nice having a fallback in case you forget to turn off the AC or heat while you're at work. Kind of like how no one really needs the Apple Watch either but it's awesome for the right customer.