Custody question

[mistoffolees]

I recently attended my court-ordered parenting course (all parents have to take it in my court district). One of the things that was very eye-opening was how much the child's need for the non-custodial parent changes over time. Generally, the younger the child, the shorter and more frequent the visitation should be.

Yet virtually every custody agreement I've seen has a set custody and visitation schedule and the only way to get it changed is for one parent to go back to court.

Why don't people write agreements that adjust over time? For example, it might say:
- the father sees the child for 2 hours three evenings a week.
- When the child turns 5, the father sees the child every Wednesday and every other weekend
- When the child turns 10, the father takes the child every other week
- When the child turns 16, the child can choose where to spend time, but a minimum of x days per month at each home

Ignoring the specific details above (which would be very different in different situations), why don't people do that? It seems to make more sense than assuming that the needs of a 2 year old will be the same for their entire life.