A variety of reasons; for example, the ubiquity of NAT plus the use of sleep modes means it's painful to get smartphone apps to talk directly to the hardware in question. Far easier and more reliable to have a hosted intermediary.
Additionally, the thermostat supports demand-response features (where your utility shuts off the thermostat), and can even pre-warm or pre-cool before the demand-response event as needed. The sprinkler controller checks the weather and works out optimal scheduling. Both receive regular firmware updates, and feed telemetrics back to the company for further improvement of their firmware.
Technically all of this could be done independent of their own hosted service, such that everything would still run normally without it. But there's a lot of connected features, and integrating a centralized hosted service simply makes everything far easier to develop.
At which point keeping a centralized service out of the equation becomes a feature requiring money & manpower to develop- a feature most customers don't care about, at that. (And let's be honest, what company wants to spend their energy on a future where they are out of business?)
Additionally, the thermostat supports demand-response features (where your utility shuts off the thermostat), and can even pre-warm or pre-cool before the demand-response event as needed. The sprinkler controller checks the weather and works out optimal scheduling. Both receive regular firmware updates, and feed telemetrics back to the company for further improvement of their firmware.
Technically all of this could be done independent of their own hosted service, such that everything would still run normally without it. But there's a lot of connected features, and integrating a centralized hosted service simply makes everything far easier to develop.
At which point keeping a centralized service out of the equation becomes a feature requiring money & manpower to develop- a feature most customers don't care about, at that. (And let's be honest, what company wants to spend their energy on a future where they are out of business?)