Because it's a customs faff. That's the point of the thread.
It wouldn't be if there was some nice website laying out all the rules in an easy-to-understand way...
Your logic is "nobody needs a site detailing customs rules because they shouldn't be traveling with the things that I personally already know might conceivably cause a customs faff". The point is the website would solve the problem.
So make it easy to look up and don't let guesses at what some random country in the world might object to dominate all your travel packing and planning (Guesses because until I checked earlier I wouldn't been sure an apple would cause problems anywhere, but apparently at least Australia objects to any fruit). Which was the point of the comment you replied to.