1. It's law and it's written into the us constitution. Most constitutional amendments have been small alterations or additions to the document. Anything that fundamentally changes the structure of the us government sets a dangerous precedent - i.e. That those in power in Washington can muck around with the system. What's next - congress appoints judges instead of the president? It is a slippery-slope argument, to be sure, but the "if it ain't broke" cliché is a cliché for a reason.
2. The electoral college was designed so that a president couldn't be elected by appealing to a particular region that was denser or more highly-populated than the others. This was a big worry to the founders, who feared that a particular state or region might become so populous that it could single-handedly determine who became president. The electoral college solves this by ensuring a president is elected with a broad geographical consensus. Small states are given extra weight in order to tip the scales back in their favor.
3. The electoral college maintains the two-party system. It is very difficult for parties other than the two main left-right parties to break into national politics because of the winner-takes-all nature of the electoral college. This can be seen as a negative - good ideas and minority interests are left behind - but also as a positive. A two-party system is dynamic, and the issues over which they disagree move back and forth with the political will of the people. Politicians have to be re-elected, so a politician trying to win votes has to do it by moving to whatever position his constituents demand.
Lots of people will say this is a bad system, but it is efficient. The us government gets things done, despite all the yammering and garbage that comes emanating from Washington. Compare it, for instance, to the Italian parliament, which with 16 parties is a very open system, but there is such frequent disagreement and bickering that nothing -ever- gets done. A two-party system makes it very easy for there to be a majority consensus, and some would say this is the strength of American democracy. I leave it to you to decide.
There are a lot more pros to be considered - I would like to point you to a document written by the deputy director of the federal election commission. It is an excellent read and there is a section on pros and cons towards the end of the document. I include the link below.