While we do want to give Kamino some bonuses to make it the best place to build clones and, when resources allow it, we do want to represent other local forces more (infantry take more time than other things to make, and fewer people know how to do it properly- the first releases of the mod won't be able to have them at all for this reason, we already have tons of assets we need to make), we don't want to make clones exclusively produced on Kamino, the same way we don't make ISDs exclusively produced at Kuat or Mon Cal Cruisers exclusively produced on Mon Calamari. Most planets would produce nothing military at all. When playing against the Republic, the CIS players would be easily able to just focus on taking Kamino, and almost never see any AI-controlled Clone units. Having a faction where their signature type of unit is tied so exclusively to one planet really doesn't work that well for gameplay. It may seem more lore-friendly and interesting at first glance, but the net effect is that you'd just rarely see clones.
when there were only a few million of them according to the Kaminoans
Honestly when any source that's specifically meant to be narrative gives numbers, they're kind of meaningless until put into context later, if at all. It's been 40 years and different sources still diverge on whether the Empire had three and a half star destroyers or tens of thousands.