It is because you have to fast the 6 days of Shawwal after you have completed Ramadan, thus, you can’t get the reward of fasting shawwal while your Ramadan is still incomplete.

I have heard from one of ur class that we can combine intentions eg:-fasting on the day of Arafat with the intention of making up missed Ramadan fasts n inshahAllah getting the rewards of both..but y is this not applicable with the 6days of fasting on Shawwal.??