Why too many capacitors in parallel for Vdd supply net? Can't we just add all to replace with one big capacitor?
These caps are used as "decoupling" capacitors.  Even though they appear like they're all next to each other, they will be located (often in pairs) on the circuit board next to power pins of digital IC's.

Unlike analog circuitry, a digital circuit uses power in short, fast bursts.  All traces or wires have some inductance, which prevents the current from changing as quickly as the IC needs it.  This causes two problems: The voltage fluctuates at the input pin, and the rapidly-changing current causes the traces to radiate electrical noise.

A decoupling capacitor provides two main functions:


The first function is to prevent these two problems.  It acts as a small power buffer right at the IC, and can provide the necessary rapidly-fluctuating currents.  Since they are located right next to the IC's, there are no long traces to act as noise generators.
The second function is to act as a filter, dampening noise seen from the outside of the chip.  This is where the multiple values of capacitors come into play.  The capacitors have some small parasitic inductance, also.  Each capacitor you add creates an LC filter.  Each different capacitor value, combined with the parasitic inductance, filters a different range of frequencies.  It is common to see a 100pF next to a 0.1uF cap at each power pin.  This combination has a favorable filtering bandwidth.


So, even though you could use one large capacitor to match the nominal bus capacitance, you would lose the decoupling benefits.