Why?
Seems a reasonable question – even if the practicalities are starkly obvious…
At one of our previous addresses we very rarely received any mail that looked remotely like birthday cards.
After one birthday (my son's second?) went by with no cards / pressies we checked with relatives as to what had been sent. Only fairly little gifts etc, but the conclusion was obvious – some thieving scumbag was siphoning anything interesting looking out of the mail.
We complained to the appropriate bits of Royal Mail and basically got laughed at. A few years later there was a local TV news clip about an invetigation and prosecution of local delivery team members who had been systematically "redistributing" the mail… 👿
Frankly, someone should take these sort of issues more seriously – and if it needs Police action to increase the profile of this issue, then so be it. RM management seem to accept that it will happen – and based on a few reent TV "investigations"* some RM personnel seem to regard it as some sort of unofficial bonus scheme.
* May or may not be true depending on TV company's susceptibility to over hype their own investigations and findings…