IMHO, it’s the fact that there’s a (basically, by default) a single market driven by (basically, by default) a single search engine to locate software. The desktop market, is largely NOT driven by a single market or search engine. With the “single market” if a malicious actor can game the search engine for even a day, they can install on (potentially) millions of devices. However, with the ‘ole distribution model, it’s much harder to game.
With scale comes new challenges and attack vectors.
This is most likely because people do not have to buy a PC every 3 years. Phones are treated as disposable items, whereas with a desktop, you can open it, clean it, change parts etc. I can even change the CPU in my laptop if I wanted to.
We can assume both as having reached steady state and that’s mostly replacement rate (not quite but almost).
Cell phones are more popular for obvious reasons: full access to internet at all times, TCO is drastically smaller across the board (upfront cheaper, cheaper internet, etc), and entire structures are built around them (many critical services go app first if not even app only).
I think you’re applying a very western bias in your analysis.
I think this may be a case of rose-colored glasses. How many PCs were and are a complete mess because Windows both is a free-for-all and is (or was) the largest attack target? Sure, we know how to keep our desktop systems sane, but most people don't.
Goggle's interests are aligned with the surveillance capitalists, Apple is either competitive (see Facebook) or ambivalent.
Apple can differentiate itself by trying to sell the security angle and hurting the surveillance of people in their app ecosystem doesn't hurt them. Comparatively, I would say Apple has a nontrivial advantage in security, though I wouldn't make a broad statement that iOS is "secure".