There is an channel configuration option to automatically join people to the channel.
See: 📣 Introducing default chat channels (automatically add users)
This adds everyone with access to the category a channel is associated with, so you have to be somewhat more intentional with your category/group/channel design.
The idea of simplifying things and removing the category indirection certainly comes up from time to time as well, and it may be something we revisit at some point too, but we don’t currently have any immediate plans to change that.