With my business hat on, I'm not so sure. "Why are we spending development resources on a feature that isn't valuable for our target users?"
I could definitely see doing this if it were a product with a prosumer-type angle to the value prop. But for the devices we see on the shelf at a big-box store, I don't think those companies' management considers that valuable.
When the Chamberlain story happened, I received questions from executives on why it was such a big story.
From a business perspective, it just isn't.
I agree with you.
Same reason we don't dedicate people to write HA integrations.
1. Making sure their app can control the smart devices even while the internet connection is out as long as the phone and the devices is connected to the same lan (local control). Local control adds resiliency to your product, which increase user satisfaction. Don't see it as spending an effort to support home assistant, but instead, see it as making your own product more resilient to unstable internet connection.
2. If your device don't support ZigBee (or other local protocol) and only supports wifi, have the local control api secured with a key. This key will be generated during initial setup and should be retrievable from your app.
That's it. If your devices are popular enough, someone will poke around, see the device has local control api secured with a key that can be retrieved from the official app, and publish an open source integration on HACS. You spent zero effort to directly support home assistant but your users now has an option to use their devices with home assistant and will likely to be a repeat customers.
Which is why they aren't.