Because they don’t want people approaching immigration having not filled it in? Will lead to absolute chaos if people are filling it in when queuing for immigration. Those that haven’t filled it in will be directed to the naughty corner to fill it in.
It’s bad English on website I’m assuming . Should say within 3 days in advance of arrival. Meaning you have maximum 72 hours to apply but can still apply on day of travel. This is the way the Philippines entry works too.