Why doesn’t Facebook allow for CPI optimization at the start of a campaign?

You need to generate some number of installs before Facebook will allow you to start optimizing for install events. After a while, you'll see "Installs" in the list of events for which you are charged.

With no information about how your ads perform with respect to driving installs -- that is, Facebook doesn't yet know how many times it will have to show your ad per each install generated -- Facebook is only willing to charge you on the basis of impressions. Once it gathers more data about your ads' performance for the defined audience segment, it can estimate a conversion rate and work that into a delivery volume.