Not even "seems like," they literally were publisher mandated. DS was in development even while DeS was still deemed a critical and financial failure by Sony. Bandai Namco agreed to fund and intl. publish DS under the condition that it become a trilogy. DS2 started development *before* DS1 released.

The only real graphical distinction between PS2/Xbox and PS3/360 I can think of is with regards to visual effects and lighting, although even those are still somewhat hard to discern between generations.