Bots are broken as a platform. No advertising, no payments system, developers have to pay for hosting, there's no discovery mechanism because the messengers don't have bot stores... Why would developers be interested?
For exactly the reasons you mentioned.
Now what would business folks be interested? That's another question.
There also are bot stores on some messaging apps. I know Skype just added one. And facebook and telegram were giving out grants for users to develop interesting bots on their platforms.
Also bot functions can be federated and forwarded quite easily.