Because they are constant current > constant voltage regulators and set not to... If you are only presenting the battery with 13.XX float volts that is all you are presenting the battery with and you can't over charge.

The battery will only accept X current at X SOC and X volts. The only thing that can cause over charging is by changing the set point voltage back to absorption and leaving it there. For example if you held the battery terminal voltage at 13.4V with a chock full battery, and put 1,000 A charge source behind it, at 100% SOC the battery will still only take perhaps 0.08A. Now put 300,000 A behind it, at the same SOC and same voltage and it still takes 0.08A. Conversely put a 2A solar panel behind or ten with ten controllers, at the same parameters, and guess what it still takes the same current at that voltage.

Beyond that if your controller gets to float and can maintain 13.XX volts then you will likely only have one controller active at a time due to very slight differences in voltage set points....