Because they're quick/easy/fun. My main requirement was a RAD environment to quickly spin up GUI apps for my family - apps which are reasonably small and memory efficient (so no Java / Electron etc).
In the case of GAMBAS, I originally picked it up because I missed the days I used to code in VB6. I recall chruning out small utilities like it was no big deal (and I was still in school at the time)! So GAMBAS, as a spiritual successor to VB6, was a natural choice for me - and it's also cross-platform, which is great because some of my family members use Linux, whereas others use Windows.
Before GAMBAS I used to use Delphi, which had a similar RAD environment with Lazarus. But Lazarus is a bit clunky, compared to the polished experience of Embarcadero Delphi (which is Windows-only, unless you pay for cross-platform). So when GAMBAS came along, I first played around with it just for fun, but then over the course of time it became pretty solid and capable, so I switched to it full-time for my GUI needs.