Why does the rate of temperature change does not stay constant over time

If you graph the heat added to a system versus the system's temperature, the graph usually slopes upward; adding heat increases temperature.

The rate of temperature changes and doesn't stay constant over time becasue the temperature T(t) is not a linear function of time. This means that if the object is initially at 10 degrees and the atmosphere is at 30 degrees, then time taken to increase from 10 to 12 degrees will be different to the time taken to increase temperature from 12 to 14 degrees and so on.

answer : a model is often used to explain and make things easier to understand when any scientific topic needs to be demonstrated and explained.

as in this case where a scientist wants to use a model to present the results of his detailed scientific investigation. he should do it as it will him to make the audience understand the concept in a much better way. also, it can be changed according to new scientific understandings.