Because you are able to build and modify radios as a ham, many do and charge for said hardware. This just spilled over into software where people feel they should be paid for the work they’ve done just like the hardware folks. And since a lot of that started before OSS was popular it’s strangely stuck around.

That said some of the most popular ham software (like WSJT-X) are open source. I think the trend is starting to shift the other direction.