The issue is the phone is (almost always) always on and you cannot inspect things like you could with a PC. You can't list processes and read through them, there's no syslog or kernel log on most phones that you're allowed to view, it's a much bigger pain to reinstall than a PC is, there are a lot of things that make it an easier target. I would have a much much easier time figuring out my PC is compromised than figuring out my phone is compromised