It doesn't, in the general moral sense. Virtue and how strictly it is enforced is a pendulum, or a cycle, or a
4 generation wave, however you want to describe it. Go back 50-60 years ago and you'd be asking why society
loses morality values as the post-war order was being deconstructed.
If you mean a slow numerical accumulation of "total things to think about", that's a time function of socializing, really. One person alone doesn't think about interpersonal issues. Two people have to consider each other, three have to consider group dynamics, 10 have to consider factions, 100 have to consider sub-cultures and how to treat them, etc. Keep any of those social groups running for years, and the list of conflicts and considerations will just keep accumulating.
That also compounds when you change their living arrangements, either because of technology or population size. 100 people living in a village are less tightly coupled than 100 people living in an apartment building. The closer you are forced together, the more interactions you have with others, the more concerns will be raised. The threshold for you doing something (or not doing something) affecting a neighbor decrease, and the effects increase. The rules, conventions, and considerations needed to guide those interactions necessarily increase.