That's because building software & running tests is somewhat standardized, but deploying is very specific to each application's environment (and requires additional credentials, integration with monitoring, automated rollback, etc).

Some platforms provide this as part of their feature set (like AWS).

You talk about your side-projects - how do you deploy them today? Write a script for that and think about how many unique-to-yourself edge cases you are handling. If you believe your solution is generic and re-usable, then build that into a tool/platform for everyone and profit!