Just out of curiosity, what stops regents from making all thier
provinces level 10?

It doesn't costy that much.

To make a third level province into a tenth level costs a meger 39 GBs
If one rule action was used each turn for 7 turns it would average out
to-5.5 GBs per turn, not much.

3 to 4 - 4GB
4 to 5 - 5 GB
5 to 6 - 6 GB
6 to 7 - 7 GB
7 to 8 - 8 GB
8 to 9 - 9 GB
9 to 10 - 10 GB

Total-49 GBs