Real-world attacks aren't normally done in a manner that will tell you you've been attacked. Ransomware is an exception, and pretty much the only one that tells you "you've been hacked".
After that, its hoping malicious software scans pick up something - but anyone competent will already have verified that their code doesn't trigger those detections. Once they do, those attacks stop being used because you do not want to tip off your target that you are trying to break in.
Some attacks are after information on your device. Whether thats passwords, bank account details, or confidential information on the upcoming iPhone. So they harvest that, and stay quiet. Some attackers will even purge themselves from your system once they're done.
But many also use your device as a staging ground for attacks against other systems. Systems that may otherwise be inaccessible.
Note: Those systems don't have to be of the same type. An iOS device could be used to gain access to an internal Linux or Windows server, or the control software of a Nuclear plant.