Snow is localised and melts into captured streams. It gets refilled because high alt + low temp preferentially causes rainfall/fog that replenishes the snowpack.

If fewer rain cloud condense on the peak then it just rains more dispersed which leads to much less efficient capture (some absorbed by the ground, some rains over ocean, evaporation is faster when you have water spread over large area so fraction of time spent in liquid form is smaller, etc).

It makes a big difference. Imagine watering your favourite plant once per month with 4 litres of water, vs watering weekly with 1 litre of water. The snowpack melt cycle allows for gradual watering of the surrounding areas. Not directly answering the question for reservoirs, but the rapid loss of snowpack and glaciers has a massive impact on the environment.

So that a system of dams that works for snow pack would need to be scaled up five x or ten x to collect all that flow in less time.

Right. Snowpack is a form of massive reservoir. Snowmelt through the course of the year allows us to constantly release water from human-made reservoirs while their levels stay the same. There is still snow in the Sierra today, and in a typical year snow stops falling in April.

Rainfall is much, much harder to capture, and the release curve is completely different; you can very quickly go from flooding to bone-dry.