>>520718027
It's the state's decision. Some states act do split their EC votes based on proportion, but most have chosen not to.
Solidly Blue or Red states are in a game of chicken; if California were to split EC votes proportionally without an equivalent number of red state EC votes being split in that manner, it would simply weaken the Democrats. Swing states actively prefer the arrangement because splitting the EC vote in their state would render them less important and this they'd get less government pork.
Originally the system was designed before the existence of instant, long distance communications. The Electoral College served a necessary function. If it takes several weeks to reach DC to tell everyone how your state voted, the situation could have changed.
Right now, blue states tend to prefer proportionalizing the electoral college, while Red states prefer the status quo. This is because the electoral college favors Republicans. This is a function of the current structure of American politics. If the Democratic coalition were to collapse(as seems possible), there could be a new web of political alliances and the math could change once the Democrats recover, or a successor rises in their place with a new coalition.