Do you have a problem with legislation that stops tax evasion, money laundering and other criminal activities?
Not at all however there are many perfectly legitimate reasons for ordinary folk to want to use cash for certain things.
Here's a hypothetical scenario for you: Imagine a member of your family suddenly found themselves in money trouble and desperately in need of a couple of thousand pounds to tide them over, then lets say that was money you had in the bank and were more than willing to gift to them.
Now think how miffed you would you be if the rules prevented you withdrawing that and simply giving it to them and that your only legal options were by cheque or some sort of electronic means both of which would leave a trail which in turn could potentially result in them paying up to 40% tax on it as unearned income ?
Giving it in cash would be evading that possibility but would it really be tax evasion in the sense that you are arguing, I think not.
There is also stuff like buying a second hand car where both for buyer and seller folding money will always be king and with a little fore though and planning that's likely to remain so regardless of any rules to the contrary.
Finally it's a rare individual indeed who can honestly assert that they have never so much as once cut a deal with somebody for cash so be very careful when climbing up onto that pedestal !
Sadly, and as in so many other fields of regulation, the rules often harm and inconvenience the innocent as much or more than the guilty who will always find alternative ways to conduct their nefarious businesses.