Need clarification why Depreciation expense account is not defaulted through Asset category

Need clarification why Depreciation expense account is not defaulted through Asset category - while creating the asset through Mass Addition Window changing the queue name to POST or uploading through Spreadsheet, we could see the Expense Account is mandate field. As per standard functionality, both Cost & Clearing account are defaulted through the asset category. but the Expense account is a mandate field as and when we create a asset record be it in Mass addition window or through Spreadsheet. so wanted to understand why.?