Why is METAR QNH, not acceptable for being used for instrument approaches, as per the ENR1.5-37.

I can understand in years gone by, when met observations were done manually at set times during the day and night.

With observers replaced by AWIS’s, and that information fed into the BOM computer system and then onto Air Services- the QNH should be reasonably accurate.

Could be up to 30 minutes old is my guess.

What if we changed METARs to being issued every 15mins. Or is that too logical?

Nope. You might be listening to a 14 minute old observation that’s about to change.
“An actual aerodrome QNH obtained from an approved source is valid for a period of 15 minutes from the time of receipt.”
You would have to regulate that the reduced minima is only available until the end of each quarter hour when you must either get a new forecast, listen to the AERIS (if it covers that aerodrome) again, or ask flightwatch for another update. Easier to just listen to the AWIS and note the time.
Interesting to note an Approved Meteorological Observer isn’t on the list...

And the AWIS could change a minute after I got it’s QNH. No real difference.

I am sure there is a safe, logical way to be able to implement use of METAR QNH.

You won’t find a 2 HPa difference occurring one minute after your last AWIS receipt. Keep in mind over a 30 minute period, a 2 HPa difference will trigger a SPECI, but you still can’t legally use it.

ANYTHING is more accurate than an area QNH...lol