Something like a default Ubuntu install (which might be somewhat "mainstream") actually has a surprising amount of stuff running in the background. Of course it's all open-source or otherwise completely documented, so it's still easier to figure out what's what than with Windows or macOS.

It can be lighter but what does that mean in practical terms if the cost of maintenance is monumental? I think with even just the barest bones practical computer with wifi + with email + browser + compiler and their dependencies is well beyond the scope of what one person is able to audit. You'd need a team of at least 20-30 individuals before that starts making sense.