They are exceptionally rare, but it does happen.
In essence, each car in the game has a numerical, four-digit "Car ID". I think that what happens is that the Daily Workout has a giant (randomised, but not random) list of numbers from 1000 to 9999, running through the sequence over and over again. When you meet the criteria (driving 26.2 miles in one calendar day), the next number on the list is pulled out, matched to a valid Car ID, and that's the car you get.
Of course there's only 340 valid Car IDs, so most numbers don't match to a Car ID. What probably happens if a number that doesn't match comes out is that it looks for the next number up that is a valid Car ID. That would account for the frequency of some cars being much higher, and it would allow for cars to be added to the list.
Imagine if the game only had two cars and the list was only ten numbers long. You'd have something like this:
01 - Car 1
02 - (blank)
03 - (blank)
04 - (blank)
05 - (blank)
06 - (blank)
07 - (blank)
08 - Car 2
09 - (blank)
10 - (blank)
If it looks up 01, you get Car 1. If it looks up 08, you get Car 2. Any number from 02-07 is invalid, so it looks for the next number up that returns as valid, so you get 08 and Car 2. 09 and 10 are invalid, so the next number up is 01, which is Car 1.
That means 70% of the DWG cars would be Car 2 and 30% would be Car 1 (which looks like a bias towards Car 2, but it isn't).
The game is then updated to get Car 3:
01 - Car 1
02 - (blank)
03 - (blank)
04 - (blank)
05 - (blank)
06 - Car 3
07 - (blank)
08 - Car 2
09 - (blank)
10 - (blank)
Now you'd get Car 3 for anything from 02-06, Car 2 for 07 & 08, and Car 1 for 09, 10, and 01. That would mean 50% of DWGs are Car 3 suddenly, while Car 2 drops to 20% and Car 1 stays at 30%.
Imagine that, but with 340 valid four-digit IDs and a list of 8999 numbers repeating a quarter of a million times.
If the 20m credit cars have a Car ID that is one digit higher than any other valid Car ID, they would appear literally once each every 9,000 days - more than 24 years - for you.