Many reasons, but one is that for a high-powered easy to customize
semi-auto rifle that shoots a high speed round, they are...cheap when
new and even cheaper used, and in many states you can buy one privately
without any background check.
Didn't you put one of those together? Do you still have it?
I lightly modified a Colt HBAR with a different handguard, gas block,
and an ambi safety, but I traded that rifle in years ago to an FFL
dealer. My changes did not change the function of the firing mechanisms.