why high current can't substitute low voltage.
There are situations where high current DOES substitute for
low voltage, like in a transformer. But, all real-world applications
have current limits, voltage limits, and thresholds. A wire that
carries 15A of current is thick; hard to bend, expensive to build from copper, and requires good low-resistance terminations to connect it
to anything. A wire that carries 30 kV to fire a spark plug
is always fat with insulation, so that it doesn't make St. Elmo's fire
(corona discharge) when active.
I have a 6V 100W light bulb: it takes about 15A to run. And, a 120V 100W
light bulb takes 0.8A. They are very different designs, and extending
to 0.6V or 1200V would require very different switches, wires, sockets.
let's say a diode, I need 0.7V to make it operate, why if I decrease the voltage below that and significantly increase the current it still doesn't operate?
A diode that does useful rectification of electrical power, made of
silicon, has negligible power output with less bias voltage than about
0.5V. There are some RF rectifiers (point-contact diodes) that can
work at much lower voltages, but burn up at high current. So,
the lower-voltage/higher-current options are impractical. This
is because there is a material property (the silicon bandgap) that
determines the useful range for the (easy to produce) silicon rectifier.
The only way you could reduce voltage AND increase current would require a redesign of the diode itself, rather than using off-the-shelf components. Folk who DO redesign circuitry often find that their
goals are inconsistent with low currents (because cosmic rays induce
noise) or with low voltages (because transistor switch thresholds
have as much variability as the total power voltage), or with
high currents (because wiring gets too fat) or high voltages (because
insulation cannot be assured). A typical CPU chip needs
two or three precise voltages in order to function, and those
voltages are NOT arbitrary.