Why we need 100n cap parallel to IC power supply
They are needed to supply very sudden high current changes to the IC. Especially CMOS ICs may need high current changes during switching from Low to High or vice versa. If those capacitors are not very close to the IC the line between them would represent a resistance and inductance ("impedance") which impedes quick current changes. That's why they can not be collected and located some where else as one big capacitor.
It is not easy to integrate large capacitances. It is also more flexible to have the circuit designer decide how big the capacitor needs to be. Its value may depend of the application.