That's because they don't. Google takes security seriously. There's a reason GrapheneOS is only supported on Pixel devices currently as well, because of certain hardware security features.
Nothing you do with Google is private from Google but it's certainly designed to belong only to Google, your data is one of their most important assets. Of course they are going to secure it and prevent others besides themselves from getting or using it.
It's the most common misconception with Google, that they "sell your information." They don't, they never have. They use your info, aggregated with all other Google users, to sell targeting for ads. They don't sell the actual data.
The same also goes for Apple, although Apple doesn't monetize your data as much so they collect less. They'll suck up all kinds of data out of your devices but will strictly protect that data from third party applications any way they can. They're also willing to use that protection to prevent interoperability or integration with third-party devices.
AirDrop is a peer-to-peer protocol, both the recipient and initiator need to explicitly take action, and even in Apple's implementation provides no authentication (recipient device is chosen by name, which anyone can change in their settings app). There is no way the existence of this Android client would reduce Airdrop security on iOS.
Do you also believe that TLS between an Apple device and a Windows device not secure either, since the Windows device uses a different, non-Apple-sanctioned TLS implementation, and the mere existence of which would somehow weaken Apple's TLS stack?
