Because the app (assuming it is functioning) - designed to make life easier for non-EU passport holders, apparently - is quite separate to bods’ moans, joys, questions and Lord knows what else * about the EES entry and / or exit system.
At the moment the app still only seems to function in Sweden (not on many punters’ list of destinations) and Portugal, which is perhaps more popular as a destination. There again, it’s probably only of any use if you arrive by air or (possibly) sea. For those lucky enough to hold an EU country’s passport, the app’s probably of very little interest or indeed, use.
Whether the app will ever be rolled out to embrace all the EU countries remains to be seen.
* Up to and including a friend of someone being ticked off (by UK immigration) for overstaying in the EU.