You are discounting the complexity of the logistics required for an AK47 army. You need ammo, spare parts, lubricant and cleaning tools. You need a factory to build the weapon, and churn out ammunition.
Or, gather a group of people, tell them to find a rock, and go bash the other sides head.
Complexity of logistics applies to any large army. The single biggest limiting factor for most of history has been the need to either carry your own food, or find it in the field. This is why large-scale military violence requires states.
It should be noted that the purported advantages of AK action over its competitors in this regard are rather drastically overstated in popular culture. E.g. take a look at these two vids showing how AK vs AR-15 handle lots of mud:
As far as cleaning, AK, like many guns of that era, carries its own cleaning & maintenance toolkit inside the gun. Although it is a bit unusual in that regard in that this kit is, in fact, sufficient to remove any part of the gun that is not permanently attached. Which is to say, AK can be serviced in the field, without an armory, to a greater extent than most other options.
But the main reason why it's so popular isn't so much because of any of that, but rather because it's very cheap to produce at scale, and China especially has been producing millions of AKs specifically to dump them in Africa, Middle East etc. But where large quantities of other firearms are available for whatever reason, you see them used just as much - e.g. Taliban has been rocking a lot of M4 and M16 since US left a lot of stocks behind.
The only small arms cartridge plant that Ukraine had originally was in Luhansk, so it got captured even before 2022. It's only this year that they've got a new plant operational, but it produces both 5.45 and 5.56.
And Western supplies are mostly 5.56 for obvious reasons, although there are some exceptions - mostly countries that have switched fairly late and still have substantial stocks of 5.45, such as Bulgaria. But those are also limited in quantity.
So in practice it's not quite so simple, and Ukraine seems to be aiming for 5.56 as their primary cartridge long-term, specifically so that it's easier for Western countries to supply them with guns and ammo.
If you think the AKs in use in Russia and Ukraine aren’t getting regular maintenance, cleaning and spare parts, I don’t think you’re watching enough of the content coming out of the war zone.
Soldiering isn’t sexy, it’s digging trenches, cleaning kit, and eating concussive blasts waiting to fight or die.
You don’t sit in a bunker all day waiting to defend a trench and not clean your gun.
Or, gather a group of people, tell them to find a rock, and go bash the other sides head.