As far as I know, you can't upvote two comments in less than 5 seconds. Why don't we have such a system for up/down-voting questions? For example once you vote you can't vote again for 10 or 15 seconds later. Doesn't that reduce serial voting?
A main reason, AFAIK, for this delay for comment voting is rather 'technical'. By which I mean it is (also) there to prevent somebody automatically attacking the site with extreme numbers of votes. Note for example that you also cannot fetch comments too quickly in succession. This is irrelevant for the other voting as there there is a global limitation on your votes and thus this local one is not necessary to prevent such an attack.
To prevent too much voting on comments is perhaps also a consideration, but I think a secondary one.
When people talk about problems with serial voting I think most mean the problem of one user voting many posts of some particular other user. To do this, typically, there is some inevitable small delay, like, visit post, vote, go back to userpage, visit another post, vote. So that might be done more quickly than in 15 seconds but than perhaps not even this is clear.
In addition, there is an argument to be made that it is actually better not to have some weak measures against serial voting (like such a delay) because as they are trivial to get around people will do yet then the serial voting gets harder to distinguish from legit voting.