Because the target audience for the feature is not tech savvy people but common users whom won't know it exists until it is shown to them/might be intimidated to delve onto FF settings
If you are tech savvy, you are then expected to be able to "bear the burden" of turning the feature off if it bothers you
Hell, I'm tech savvy - not a tech worker, but you'd better believe that you want me to be your end-user contact, I know a hell of a lot more than the people I work with - and I didn't even know this was an option. I'm not afraid of fixing FF settings, done it plenty of times. It's on by default. If someone who can install OpenBSD and make it a router for DSL over PPPoE in 2001 (side job) doesn't even know it exists and eats cycles [i.e., a "prosumer", not an expert, but not too far below a new hire and well beyond the masses), it's a bad idea. I don't have time to stay up on every way that people want to eat my electricity. I do know that YouTube spins up the fan on my iMac with disturbing regularity in a way that videos from alternative sources do not. So it's not the decoding.
If you are tech savvy, you are then expected to be able to "bear the burden" of turning the feature off if it bothers you