It's for the same reason most insurance policies have a waiting period. To prevent the most low-hanging fruit of fraud.
I've actually seen this happen in big cities where they have same-day popup registration tents in the middle of skid row, signing up random homeless people who don't even have stable addresses. No verification, nothing, we'll worry about that later here take a ballot. I'm sure everything was on the up-and-up there.
And, no, I’m not going to “trust me, I’ve seen it before. [I just have no evidence.”
How are they supposed to get whatever form of ID you think they should present in order to vote?
California does count ballots slowly, but it has for decades, why is it such a big issue, all of a sudden? California does lots of things slowly, is this the one we should focus on?