Because the people who need it most don't actually need it that much, otherwise they would have written it. When I debug most Go programs, I'm doing high-level debugging. A debugger is a sharp tool. For 90% of my debugging task I don't need scalpels, I need tools that can provide a holistic view of a system from above. I use DTrace for that.

When I actually use a debugger, it's low-level work; I either debug the runtime or the compiler and I use assembly. Mdb/gdb/acid is perfectly adequate for that. A debugger with special Go support is of no use to me.

That being said, a Go debugger is on the table.

Yes, on the Internet everyone wants everything, generics, IDE, higher order metaprogramming, etc. In the end, people who actually implement any tool, feature, or process are doing it only because it helps their concrete work in a very practical way. Yes, a debugger would be useful, but almost anything is useful in isolation. So far, the people who do the work didn't write a debugger, because even if it might have been useful to some, it would have been a net loss for their particular problem they were solving at that particular time, because of the additional effort.

I'm writing the arm64 compiler. I wrote a CPU simulator and a debugger. It's a very low-level tool and very useful to me at this particular stage in the port. Even though it took time to write it, it saves time overall. It's useless for 99.99% of other Go programmers.