Because it's not particularly easy to do, and there are a lot of caveats and drawbacks.
Let me rephrase your assumption: "Why couldn't you just mail me the letter in a 100lb safe. In 2022, that's my standard expectation from any service".
So - are you willing to pay to ship 100lbs for every letter you send? Are you meticulously managing the details of how to handle locking and unlocking that safe? Are you working out the details on recovery and storage, handling lost devices, configuring a communication channel for sharing certs/keys, managing several crypto dependencies and libraries - all so that you can go "Hey - what's up!" in a notification to your phone?
Or should you just stop whining - accept that this is free - and take the authors advice and host it yourself?