Why does load shedding still occur in jetliners (e.g., 787) when there is so much power available?
Load shedding always occurs during engine start, according to the FCOM of the B789. Both engines are allowed to start simultaneously. The B789 engines are started using both VFSGs of each engine, which are mechanically connected to the N2 shaft via the accessory gearbox. So, the VFSGs are using power instead of delivering during engine start. The APU cannot provide enough power to prevent load shedding.

If the engines are started using external power, at least two 90 kVA external power sources are required. Optimal start performance is achieved using 3 external power sources (2 on the left forward fuselage and 1 behind the left wing on the fuselage). If only 2 external power sources are used, significant load shedding can occur (e.g. First Officers displays blanks and even the audio control panel receive and trasmit selections may be lost).

The APU is normally switched off during flight, unless e.g. one VFSG is U/S.

Load shedding should normally not occur when both engines are running and all VFSGs are operational (I haven’t experienced load shedding during flight).