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.
If only it was possible for massive armies of people to inspect source code for possible defects or backdoors?
Further what if it happened we were unsatisfied with this we could all collectively hire more people to audit the software stacks we rely on in order of priority instead of expecting each person to hire dozens to vet the software they are presently running.
Further if only even if we can't ever arrive at 100% surety we can get closer and closer to satisfaction.