I suggest there are three issues when comparing purpose-built hardware wallet with DIY

1. many users consider unsafe things to be safe, such as airplane mode on a phone

2. many things are not absolute, but a matter of degree, especially the risk of malware from the factory on a USB thumb drive or a SD card

3. DIY is a time sink, requires time-consuming problem solving for relatively minor issues. The temptation to save time by just connecting to the network for 2 minutes creates a risk which doesn't exist if the user just pays the money for a purpose-built device