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.