Nobody never said they would be. However, their market share is notably smaller (and the operating systems they run on is vastly more diverse), and thus there's little sense in writing bulk malware targeting them. Targeted attacks definitely, though in that situation one would be screwed almost certainly anyway if the adversary is capable enough.
And pretty much most of these exploits are targeted attacks already.
The better they get at emulating quirks of Office the more open to this kind of stuff they get.