Extreme disproportion in the dispersion of "Digital Root" in the Collatz Conjuncture highest numbers reached

I calculated the Digital Root for the highest numbers reached in Collatz Conjuncture. for example for 39 as the seed number, the highest number reached is 304, so the Digital Root is 7.

what I found is that there is a huge disporportion in the numbers, most of them being 7. In the first 100 seed numbers, 65 of them give 7.

even for random numbers I checked, they mostly led to 7 (for example 111, 222, 333, 444, 555, 666, 777, 888, 999 all lead to 7)

since for a normal set of natural numbers, the Digital Root would be porportionally distributed between 1 and 9, I can't find any reason for this.

can anyone explain this? or is there any text about this? has anyone checked this before?