There plentiful because some many people where looking at them, because they where so expensive in the past IMO. There are other advantages as well, less device fragmentation makes it a lot easier to sell exploits that work on all phones.

I would not be surprised if over all android phones where less secure. One of the reasons being the device fragmentation.

Apple has also recently taken decent strides to make one of the main 0 click vectors (Messages) harder to exploit with iOS 14.