Why aren't AFCI breakers required for bathrooms/lavatories?

NEC doesn't require AFCI because it does require GFCI, and they are not the same. GFCI outlets protect against electrical shock and this is very important around water.

AFCI protects against electrical arcs that come from damaged cords and bad connections. Arcing is super hot, and is responsible for electrical fires.

A combination protection device could be installed in the bathroom, but some people say the combo's don't work quite as well as single purpose GFCI, and arcing fires are generally not a risk in a room filled with tile and other hard surfaces.

1. Maybe because arc faults are not more dangerous in those places then in other rooms of a building. The probability of arc faults to go undetected may be even lower in bathrooms, since normally all electric appliances and devices are only switched on if that room is occupied. An arc could be detected by smell, flickering or fume. A fridgerator, A/C unit, laptops and other devices are often running in rooms without a person being present all the time.
2. No, assuming there is a GFCI and a correct earthing. In that case, an AFCI protection is as indicated as in any other room. In theory, the probability for an arc to occur increases with the water in the air, but a room's air without any water outlet could have a high moisture level as well, dependent on location and weather. Otherwise, a GFCI is very important in rooms with water and electric outlets, independent on local codes.

We're reading tea leaves here to guess at NFPA's intent. NFPA writes the "model electrical code" which they offer for anyone in the world to adopt as their law.

But politically, NFPA has been having a big problem. Normally NEC changes are fairly trivial in cost: Pull a neutral wire on switch loops, gosh, you're using the /3 Romex instead of the /2. It's maybe $3 per house. Those increments have been getting bigger, as they've started so snake GFCIs and AFCIs in more and more places.

Mind you, AFCIs started out as a solution for electric blankets starting fires in bedding. However, they discovered that in actuality, AFCIs were catching arc faults caused by backstab connections. Backstabs are a builder favorite (jab a wire in a hole and you're done, instead of having to form a shepherd's hook and torque to spec, times say 200 connections in a typical house). Builders don't want to quit backstabs. (if they fail after closing, who cares?)

Well, in 2014, NFPA was hellbound and determined to require AFCI or GFCI breakers on darn near everything. And a lot of people were thinking "Why not just outlaw backstabs, then?" Anyway, NEC 2014 was adding six hundred bucks to the cost of a house, which was actually going to have a material effect on real estate prices.

Builders were saying "Oh hell no", and telling their Congresspeople to say "Oh hell no". And NFPA had to lobby right back, and influence public opinion. So states adopted NEC 2014 very reluctantly (seven still haven't), and some states still adopted it with some AFCI/GFCI requirements left out.

Anyway, requiring both AFCI and GFCI on a circuit was politically "a bridge too far".