Why is it so difficult to control IoT devices from your desktop?

Smart home device manufacturers want you to use their apps. Over time many have become increasingly hostile towards open source solutions such as Home Assistant.

I was an early adopter for Smart Home products. Some early products could be made to work with Home Assistant and my in-house server. But more recently devices are harder to make work. I have ripped out many of my early attempts.

Most certainly I wouldn't trust any of the smart locks. Exploits are being found and published.

You'll get better responses if you add more context. As it is, this isn't worth trying to answer because it's not clear what you're trying to do/say or what things you tried that didn't work.

They don't want to have two different code paths (inside the home vs. outside) and some home networks are broken (e.g. wired clients can't contact wireless) so they just implement cloud-based management.

use https://www.home-assistant.io/ ?