Apparently integrating with "legacy" systems was a big bottleneck (e.g. to verify your SSN). I don't understand why this verification had to be done "live" tho

Apparently integrating with "legacy" systems was a big bottleneck (e.g. to verify your SSN). I don't understand why this verification had to be done "live" though; the obvious alternative would be to collect all the data, verify it asynchronously/overnight, and then flag up those people that had issues. You enter your birthday, SSN, etc and then the 1% of people that got their details wrong get an email the next day saying "we weren't able to verify your details, please log in and fix it".