You are right here but in most cases other than bitcoin, we do not have many choices to install the wallet on the computer. For example, one of the trusted wallet for storing bitcoin / Altcoins is Unstoppable Wallet and it does not have a desktop version. Also, another popular but closed source wallet is Trust Wallet and it also does not have any desktop app.
Trust Wallet doesn't have a desktop app, but it does have a Chrome browser extension for those who want to use it on their computers. You are right about Unstoppable Wallet, though.So, if the phones were so vulnerable, these wallet developers should also focus on developing the desktop versions too 
They are vulnerable if used in ways that could make them vulnerable, and modern smartphones offer that, unfortunately. I mentioned some of the ways in my previous post. They are not vulnerable by default. A desktop computer can be equally vulnerable if the person sitting in front of it has the wrong online habits.