Because Apple uses by default iMessage which acts more like a online messaging service, like whatsapp or telegram.
However Apple being Apple, this service only works between other Apple devices.
iMessage should default to sms when communicating with non-Apple users but it’s not always great.
It’s a bigger problem when someone had an iPhone and switched to android, as it will attempt to direct the message to their iMessage account, and sometimes messages get lost or delayed because of this.
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