I believe the RNoAF have this requirement. We also came upon an elderly gent in VASF at Lista. He had had time promotion in that one job, I believe he was a Lt Col.
Now in the RAF, to put all JO into airmen billets to JNCO rank would displace many airmen from getting promotion. As Cpls have rather more experience than even a new flt lt, just how long would you propose that officers serve before promotion?
Pretty clearly the OP was refering to Logs and HR rather than aircrew or ops support as they rarely get close enough to airmen.
The new officer is not expected to be as proficient in the trade as the SNCO but a good one will learn quickly from that SNCO.
Many years ago it was the norm for crusty SNCOs to help and mentor plt off and fg off but to expect a flt lt to know which way was up. Since many flt lt were straight out of training this was quite difficult for the new JEngO.