When Padi required quarterly tests and some centers did them, 3% failed - a significant portion of those by large amounts. Safe...?
DAN admits that they have no idea how many drownings were caused by CO poisoning as the required testing is seldom done at many destinations and even if it is done - the findings are easy to hide in many countries to protect tourism.
Simply put, if you don't test - you do not know, just diving on hope. Inline units are $600+ USD plus frieght and shipping but the total costs spread out over the life of filling tanks is pennies/tank - not many pennies at that.
Paranoid huh?
The scuba sport & industry was certainly developed before SPGs, Octs, computers, and more recent developments of low cost, easy to use personal CO testers, so much of the industry is still stuck in bad habits with regard to air quality. And with many Instructors hiding their heads, resolving this unneeded risk is challenging.
True, the actual known body count for scuba deaths is relatively low so CO deaths must be lower than that, so if that's good enough for you - cool. I'll keep wearing safety belts in moving vehicles, float vests in moving boats, and testing my tanks before I take them to depth. The inconveniences are small and the uncommon negative results are great.