what does magnitude of shock do? and why there are so many more lightning nodes than cold and fire?

All I know till now is:

shock can have monster get 20% more all-type damage(not over time)
so with %magnitude of shock means 20%*(1+x%)damage?

shock has duration, easy to understand

electroucution need keep hitting to biuld up %, electrocution only stop monster acting but deal no damage like freeze.

chill only slow monster but deal no damage.

ingnition deal overtime damage by 20% default, so %magnitue of ignition means 20%(1+x%) just like shock do?

and why there are so many more nodes on passive tree for lightning damage?

seems unbalanced, right?

The difference is the duration. Ignite always last 4 seconds. Let's say that you have 1 attack per second and 25% chances of ignite. You have more than 93% chances to keep the ignite forever and with a minimum investment improves your dps by around 80%.
With shock realistically, you cannot expect to last more than two attacks, at least not with a heavy investment.
In theory you can combine both. Shock increases the damage that ignites. In Path of Exile 1 there were builds around this concept with the node "All damage can ignites". But in Path of Exile 2 there is no such node. The infernalist has the node "All damage counts" but it is not the same. You cannot do shock and ignite at the same time.

Magnitude of shock increases the %damage taken debuff it applies. Starts at 20% more damage taken, +50% magnitude puts it at 30% more damage taken.