I wrote an elaborate response to this, but then decided not to post it. Enough has been said already.
They weren't If anything, the tolerances are probably tighter in modern vehicles and sealants are often recommended to be used by the OEM, these days, because fitting the pieces without them is either impossible or impractical. For lack of better analogy, the windshield on new cars is held by a sealant... ๐
Of course, they were not available back in the days so all sorts of machinery was likely more prone to being not perfectly sealed.
That's true. I watched a documentary where a 50-s engine factory was mixing all sorts of additives into the engine block iron in attempts to improve its properties against cracking, warping, internal tension... Very interesting process for a lay person