Re: The folly of individuals notwithstanding...
In fact banks are wary of large cash transactions because such transactions are likely (not certainly, but likely) to be associated with crime. And they work under legislation such that if they are suspicious that this is happening then *they are legally liable* for it. So they tend to be very paranoid about such things. So they tend to require all sorts of evidence that they money is not being used for anything bad (where 'bad' includes 'tax evasion by buying things for cash', for instance: it's not just drugs &c) so they can demonstrate to the government that they did the due diligence.
You might prefer they did not do this, but I am quite pleased they do.