Why are standard costs not being created for my make items, after the costs are rolled up?

You have set the rollup scope as All Items in the cost scenario and the costs for your existing make items are rolled up.

However, some of these items don't get a standard cost because the cost planning process is optimized to prevent generating standard cost records when the rolled-up cost is the same as the last published cost of the item. This optimization reduces cluttering of the application with redundant standard costs and also prevents unnecessary standard cost adjustments.