It is not a conspiracy. Just like the iOS App Store it is for your own protection. There is no legitimate reason to run your own software on general purpose computing hardware.
Doesn't really matter /a or not, it's a ridiculously reductive and extremist position either way.
Security is about tradeoffs, most notably security vs convenience, but also many others.
Anyone who suggests that their personal preferences in tradeoffs are not just universally correct but also the only reasonable position to hold is just silly.
This rings more true than many would want to accept: Most people view and use computers as household appliances; they just use whatever is installed on it and if it breaks they go out and buy a new one.
For most people there are, in fact, no legitimate reasons to run "their own" software on "general purpose" (read: household appliance) computing hardware. Almost nobody runs custom software on their washing machine or toaster.