Minute construction errors that actually make buildings lean can't be seen without sophisticated instruments. Moreso the 'lean' due to mere distance of the observer from the building. Why don't they notice that most planes seem going down as they approach the horizon even just after taking off? Ships also appear to sink on the horizon! Why don't they watch the sunset while sitting low, then notice it rise back up slightly when they stand up? Just using this observation along with a little complex math is actually enough to calculate the earth's size!