Because it's a connecting flight and specifically sold as such by SQ. I'm happy to be corrected if the below rules do not apply to a VTL flight...but as the rules stand it's pretty clear that you would have 48hrs in advance of SQ25 to complete a PCR test as the transit in FRA is under 24hrs. And just to be clear, passengers only flying JFK-FRA would not need to furnish a PCR test.

Passengers on direct flights to Singapore​​​

The 48 hours will be counted from the scheduled departure of the flight to Singapore. For example, if the flight is scheduled to depart on 10 September 2021, the Covid-19 PCR test must be taken no earlier than 8 September 2021.​​

Passengers who are taking a connecting flight to Singapore​​​

(i) If the scheduled transit duration is within 24 hours, the 48 hours will be counted from the scheduled day of departure of the passenger’s first point of embarkation in his journey to Singapore. For example, if the passenger’s flight itinerary is Los Angeles-Tokyo-Singapore and his transit duration in Tokyo is within 24 hours, the Covid-19 PCR test must be taken within 48 hours before the scheduled day of departure of his flight from Los Angeles. Or,

(ii) If the scheduled transit duration exceeds 24 hours, the 48 hours will be counted from the scheduled day of departure of the last flight in his journey to Singapore (i.e. the flight to Singapore). For example, if the passenger’s flight itinerary is Los Angeles-Tokyo-Hong Kong-Singapore, and his total scheduled transit duration at Tokyo and Hong Kong adds up to more than 24 hours, the Covid-19 PCR test must be taken within 48 hours before the scheduled day of departure of his flight from Hong Kong.