Why is rain detrimental to the fuel economy?

Lower Temperatures are detrimental as well.

rain does add drag

 

There’s also debate if tire tread lasts longer in areas of the country that receive more rainfall. Wet roads = less friction = less tread ware?

I haven’t a clue, but maybe they could be related. More rolling resistance.

 

A huge item:

A lot of stationary water mass on the road, in the path of the tires, must be rapidly shoved out of the way. This requires continuously accelerating that considerable mass from zero to highway-ish speed, costing considerable energy or power.

If this water isn't shoved out of the way, then you will be hydroplaning, which severely compromises steering and braking.

 

From a quick skim, that looks like they tested just wet roads, and stopped testing when any rain arrived. The thickest water film under test appeared to be just 0.8 mm, which doesn't seem like enough to represent heavy rain conditions in my region.

Even so, for a tire width of 195 mm, that is a half ton of water encountered, and mostly displaced at high speed, per mile of road.

 

OP I think has a good analysis.
Apx. 15 per gallon degrease is substantial.
I don't know the answer for sure.

I'd concur the biggest factor in the decrease of tire to pavement drag/ water resistance over the excess water/ lack of oxygen in the intake.
Maybe windshield wiper use is creating more downforce LOL.

 

If rain is bad for the fuel economy, guess what happens when it turns to snow at a lower temperature. I easily see a 30mpg drop on my PP.

 

It might be--but with our backwards "distance per fuel consumed" system, the same difference in MPG at higher MPG is less than at lower MPG. Convert to GPM (or, if you want to be European about it, L/100km), and a 15 MPG difference from 80 to 65 is a difference of 0.00288 gallons consumed for every mile. If we were talking 30 to 15, the increase in fuel consumed per mile would be 0.0333--an order of magnitude larger for the same apparent difference in MPG!

 

Closely monitoring in EV mode also shows a decrease in mpge when it's raining. If it's raining hard enough water displacement by the tires is one reason. Think of driving through a deep puddle....
We'd all have to monitor the ecu's to get more of the reasons why, given all the different scenarios encountered during a heavy rain set. Than make a list of the most relevant causes.

If it were easy engineers would have already figured out how to improve mpg and mpge (which they have to a degree) for better mileage during extreme scenarios. But there is also the factor of diminishing returns and system complexity. ie: if the car wasn't getting 70 - 80 mpg typically, would the difference in mpg during a heavy rain be as noticeable to the driver? I'll hazard to guess - no.

One example I'm currently exploring, during cold soaked warmups (under 32 degrees F / 0 degrees C ) while driving, which factors contribute to both lower mpg and mpge.
One of them I believe to be relevant is thicker petroleum products increasing drag. not only in engine oil for mpg, but also in drivetrain bearing lube for mpge, especially since it's more noticeable while in EV mode..
The car just doesn't roll as easily on the flat when it's been sitting in freezing temps for a few hours of more.

There are probably a lot of reasons why heavy rain decreases mpg, much in the same way that there are a lot of reasons why extreme cold reduces mpg enough to be noticable in all ICE engines, at least all that I've ever used.

 

Ever feel how your car seems to practically stop when you hit a puddle? Make that puddle very large but very shallow and you have a similar energy cost.