Basically, why would someone consider Ecgberht of Wessex to be the first king of England? He controlled a lot of land and had other English kings as sub-kings, sure, but so did Offa of Mercia. Or is the Amazon Alexa thing just not a good source of historical information?
I think that the answer is that Alexa is not a good source of information.
Here's an older thread on the topic: https://www.reddit.com/r/AskHistorians/comments/8autqn/1_who_was_the_first_king_of_england_egbert_of/dx1qmqm/
The short answer is not to trust Alexa.
Ecgbehrt did briefly establish West Saxon control over Mercia in 830, and minted a celebratory coinage in London, but this was tenuous and short-lived, as the Anglo-Saxon Chronicle records that Wiglaf of Mercia regained his crown the following year, and Mercian control of London was swiftly reestablished. Nonetheless, this did tip the balance of power in England towards Wessex, and allow Ecgbehrt to briefly claim Bretwalda status.
While Bretwalda could.be losely interpreted to mean "King of all England", it appears to have been a largely symbolic title given to a powerful king who could secure the nominal submission of other kingdoms, and not a genuine sign of unification. To ascribe 'First King of England' status to Ecgberht based on his Bretwaldaness would be to overlook other previous Bretwaldas such as Æthelberht of Kent or Rædwald of East Anglia. It's Ecgberht's grandson, Alfred the Great, who can somewhat claim being the first King of the English, although it largely fell to his children Edward and Æthelflæd to actually establish much of that kingship, and it's Edward's son Æthelstan who is crowned the first King of England.