Because the EU wants attestation for age verification, digital surveillance and censorship, not free software platforms that wont implement any of those.
Only a few politicians, some of whom happen to be currently in power, in the EU or elsewhere, are pushing for those agendas. But fortunately, they do not represent the majority.
These things wouldn't go away if the EU or a company within the EU would provide a phone OS.
They would just serve different masters.