Actually they do, and the reason for that is precisely because they allow third-party installations on Macs, and the exact same thing would happen to iPhones if they allowed third-party installations there as well.
> Adware has become much more aggressive in 2019, heavily targeting consumer and business endpoints on Windows, Mac,
and Android devices. A new team of
the most active adware families have replaced the top adware family detections of 2018. In total, we saw approximately 24 million Windows adware detections and 30 million Mac detections. The top three consumer threat detections belonged to adware families
and the number one business detection was also adware. The number one Mac detection, an adware family called NewTab, brought in 28 million detections itself.
> Adware has become much more aggressive in 2019, heavily targeting consumer and business endpoints on Windows, Mac, and Android devices. A new team of the most active adware families have replaced the top adware family detections of 2018. In total, we saw approximately 24 million Windows adware detections and 30 million Mac detections. The top three consumer threat detections belonged to adware families and the number one business detection was also adware. The number one Mac detection, an adware family called NewTab, brought in 28 million detections itself.
https://resources.malwarebytes.com/files/2020/02/2020_State-...