@Suggy You need to go back and check your statement that task tables do not inherit new fields from their parent. This is incorrect. While @Community Alums‘s response may not have directly answered your question, if you read into it a bit, you’ll see that he is saying the same thing. 

As to why do they do this? That is the idea behind having child tables (either CI classes, or Tasks): so you do not need to redefine new attributes on all children tables. This also leads into the capability of querying across the child tables, using the same field.

For example, if you were able to define a new field on Task for tracking the building it was created in, and then needed to query for all tasks created in Building X, you could filter the task table by searching on u_building_created_in = Building X. 

If on the other hand you did it by creating the field separately in each sub-task table, your filter would need to include something along the lines of:

incident.u_building_created_in = Building X

OR

change.u_building_created_in = Building X

The same type of logic holds true for CI Class tables.