Not all medical personnel are doctors, are they? Same story here. Not everybody wants to be an officer.
Of course not all medical personnel are doctors. But that's the path of medical training that they took. As far as being a "Officer / Enlisted"; that can be a choice you make later while you decide to learn your speciality first before you mature enough to decide that you want to go the command route and go back for further training to be in the "Command Training Program".
Actually, in MY head canon, Starfleet has a dual rank structure: one for its navy, and one for marines
In MY head canon, the equivalent to the US Army / US Marines are integrated Soldier Forces and share the same Rank Structure as StarFleet. No need for memorizing Dual Rank Systems.
How you rank up in your chosen career path is a different story.
In My Head Cannon, Soldiers are part of the LISUS Force which is a Sub Section of StarFleet.
LISUS = (Land / Interior / Sea / UnderGround / Surfaces), that acronym will indicate that the UFP Soldiers, regardless of Species, will learn to fight in all those environments due to the Universe being vast and having all sorts of environments to fight in should the need arise.
My StarFleet has 3x major "Class" Roles that a new StarFleet recruit can start down:
Soldiers: Equivalent to the US Army / Marine career path, but more integrated. People on this path generally double as security officers, tactical officers, ground troops, along with several other minor job types.
Aviators: Learn to operate specific Vehicles at a high level, (e.g. the vast skill gulf between a Pro Race Car Driver and a normal daily car driver) including being combat rated to operate that class of Vehicles.
CrewFolk: Learn a speciality to perform to operate as part of the crew on a StarBase / StarShip
Of course newly accepted StarFleet Cadets can go "Multi-Class" and train across numerous specialities and be rated for multiple Professions once they exit StarFleet Academy. Ergo everybody would have their Major & Minor Job sets that falls within the initial 3 Major Classes.
(e.g. Tom Paris route, a Pilot with a minor in being a medic ~= Aviator with a minor in Field Medic training.
Julian Bashir route: Certified Doctor with a later earned minor in low level certified engineering through StarFleet Post Academy Training ~= Crewman (Medical Speciality / Doctor) with a minor in basic Engineering)
Considering Starfleet has aliens accepting a human organisation the officer/enlisted might be accepted as well. In universe not every one in Starfleet is an officer.
In My Head Cannon, the Aliens saw the dual rank structure as inefficient, confusing, and dumb. Ergo it got merged into one structure. They also accepted the human Metric system due to it's simplicity and the human 24 hr cycle for StarFleet ship operations. (24 as a mathematical number is great for dividing into even parts since it has so many options, like the number 12, it is great for sub-dividing into whole numbers. That's one of the major parts for it's acception)
However StarDate became a forced acception of "Metric Time". Ergo my version of StarDate has advanced onto Version 5.0 by the start of the of the In-Universe story which is the start of the 26th Century on Earth for story telling purposes. Ergo by Earth Year (YYYY/MM-DD) 2501/01-01 Sat 0000:00.0 GMT. My format of Global Date Presentation is the new normal for Earth Date Time Presentation with Military Time becoming the new Global Standard. Granted, by "In-Universe story" time, my version of StarDate has been accepted for over 100 years.
Humans were forced into accepting StarDate into it's various incarnations with my Version 5.0 being the current most popular version with no forseen future changes down the time line. That's with verification via communicating with Future Federation members of the UFP InterStellar Time Bureau to validate that it hasn't changed further down the Time-line for the next 1000+ millenia by communicating across time to validate that the UFP / InterStellar Time Bureau is still around and no major incidents has caused the UFP to disappear. StarDate was always one of those on-going compromises in InterStellar standards to get all UFP member worlds to agree on a common format for standardized InterStellar Time Keeping.
StarDate Ver 5.0 has this new presentation format of: +345/826¦847 which is equivalent to Earth Year 2501/01-01 Sat 0000:00.0 GMT in my Head Cannon.
StarDate = (Star Year / Star Day ¦ Star Time Unit)
NOTE: The “/” is described as “Slash” & “¦” is described as “Bar” which is short hand for (Vertical Broken Bar)
- Star Year has +/- with
- + = After Federation: After the UFP was formed/founded
- - = Before Federation: Before the UFP was formed/founded
- 1 Star Year = 1,000 Star Days
- 1 Star Day = 8.76582 Earth Hrs = 525.9492 Earth Mins = 31556.952 Earth Secs = 8 Hrs, 45 Mins, 56.952 Secs
- 1 Star Day = 1000 STU (Star Time Units)
- 1 STU = 31.556952 Earth Seconds
- 1 Star Time Unit can be split into Deci-STU, Centi-STU, Milli-STU, etc via SI Prefixes
- 1 Deci_-STU = 3.1556952 Earth Seconds
- 1 Centi-STU = 0.31556952 Earth Seconds (Around 1/3 of a second)
- 1 Milli-STU = 0.031556952 Earth Seconds (Around 1/33 of a second)
- Ergo 1 STU can have (1.###) STU and so on past the decimal by using SI Prefixes past the decimal to shrink the InterStellar version of time.