Because the more banks you have cards with, the higher your credit limit ?
Why limit yourself at 2 cards with 300k each when you can have 10 cards with 300k each ?
The credit limit offered to you by each bank is independent of the credit limit offered to you by other banks, if you have little to no outstanding balance
Also each card has different caps on cash back and different modes of cashback , for example card A provides 5% cashback on petrol, but only on weekends , limited to RM50 / month . Card B provide 5% cashback on petrol everyday , limited to RM50/month, then you pump using card B on weekdays and card A on weekends, to make sure that neither card will reach the limit
same thing with groceries. Once the cash back cap on this card runs out, use the next card