[lambda]

I think its because they grew out of the phone ROM community, which pre-Android, generally meant hacking up a binary blob and distributing them on forums. This was never quite legal, but lots of people did it anyhow. Like any community build around legal grey areas, like Xbox and PS3 modders, there's a lot more anonymity, less professionalism, and the like, than there is, say in the free software world, where people are legally, and in many cases professionally, writing software to run on their own machines.

Even once Android came out, there are enough binary blobs, like the actual phone firmware, drivers, the Play Store, the Google Apps, and so on, that a lot of the mods are just redistributions of the binary packages with a few configuration changes and some custom software on top, rather than a rebuild from source of AOSP.