Well I've now gone 2 days without Chrome re-enabling itself, and think I understand the pathology.
It seems that Google have decided that I should accept the update even if the app is disabled. As long as I don't uninstall the update it will let me keep the app disabled, but if I uninstall it it will re-enable Chrome 24 hours later and pester me to update it. Of course it used to be that disabling apps automatically uninstalled all updates, so this is two changes Google have made: disabling doesn't remove updates and the system will re-enable an app to force you to install new updates.
As I have a 128GB device (about 29GB free) the extra 40-45 MB this update uses isn't a huge problem for me. But it's still an act of absolute stupidity on Google's part: what does it matter whether a disabled app is up-to-date, since it is disabled? And re-enabling the app to push the update is supremely arrogant, especially since it doesn't disable again once it is updated, which means that if you allow auto updates (the option Google promote) the disabled app would be enabled and updated in the background with no interaction with the user, and remain that way. Plus the Play Store no longer posts notifications to let you know an app has been updated, and if that's also true with auto updates enabled then the user would not even be given an indirect, obscure clue that their choice had been overridden by Google.
So the question of the thread title is answered: it is simple arrogance by Google.