It’s definitely laborious.
As Druids / Rangers go up in HD, so too do their animal companions, and assigning more HD to them can’t be done on-the-fly.
It’s also important to make sure that the different animals have some semblance of balance, so as EO said if you make an error at that HD version, or if it’s too strong or too weak, you then have to go back up the chain.
For the time needed to be spent to do it, the reward is relatively small. That dev time could be used on building dungeons, new systems, classes, etc.
As with all these things, if anyone feels passionately enough about it and are willing to slave over a hot toolset for 40 hours to bring us new animal companions, applications for developer are always open.