Certainly monetisation comes into it but even for kit where there is no forward profit (for the vendor) or cost (for the user) it is just easier to have things talk via the cloud if there is ever going to be a chance it will be accessed off-LAN. And that's what people want to do - you don't whip out your phone to switch a device on when you're standing next to the wall switch (well, not more than a few times after you first install it). You want to turn your heated blanket on, for instance, as you're on the way home from the pub.
There is also the smarts involved - timers, device interaction, etc. Devices will have limited storage and not be privy to many things (such as what the weather is like, when sundown is, etc). Some cloud server makes that trivial, so these things are going to be talking cloud anyway and it's just too much hassle to walk the user through opening ports on a router that they're not going to do that (even for kit where they don't need to do it, because they don't want a dozen different protocols to maintain).
And we haven't looked at sending notifications to your phone without using Apple's or Google's push framework. Good luck with that, and battery life, without going off-LAN.
In the vendor's position I'd be inclined to do the same as they do. Life is just easier.