Because change is hard. I don't think anyone truly thinks it's a good way to decide the presidency, but at any given time there are unprincipled hacks who will defend it because they think it's in their partisan advantage.
The thinking behind it was that Americans wouldn't know much about the presidential candidates, but they would know who they trusted in their areas to make good decisions about it. And the expectation was that there would rarely be a candidate who wins a majority in the electoral college. If there isn't one, the election goes to the House of Representatives (the more-democratic body), which picks among the top three EC vote-getters. So they didn't anticipate parties, but they figured that the EC vote would be like a primary.
If you read through the history, you'll see two main points:
1. It never worked as intended and was poorly conceived from the start; and
2. The apologetics you see today (that it was intended to make only "swing states" matter or whatever) have nothing to do with the thinking behind it.
BTW, fun bit of history that illustrates the disingenuousness of apologists
here. If a Democratic candidate ever wins the election without winning the popular vote, the EC is dead.