There are lots of variables which cannot be controlled such as people's cavalier attitude as soon as they have had the jab. The assumption is that protection will not fall off a cliff on day 22. Pfizer won't back it as they did not trial that spacing and don't want any chances of lawsuits.
Our strategy of spreading out the doses is a calculated gamble.
Those calling for us not to is just as big a gamble if not worse.
Assume we have done about 16 million jabs (adding doses 1 and 2). If we had purely focused on giving both doses at the three week interval, there would be just over 8 million double dosed.
Which 7 million are you going to take away some protection from?