What an intriguing question you’ve got.
I do not know the exact answer but if I may attempt to explain why is that so, the receivables days are captured relative to sales since the amount owed by customer is on the basis of mark-up products sold. Whereas, inventory and payables days are captured in the balance sheet at cost price, thus using cogs as the denominator would reflect the actual number of days.
I hope that makes sense to you. Lol!