I think the "LEGO tax" is as much about consistency as marketing inertia. With name-brand LEGO, you know what you're going to get quality-wise. Alternative brands can be unpredictable. I would know; I still have all my LEGO from when I was a kid, and since my family isn't made of money, a lot of them aren't actually LEGO.
Alternative brands can VERY hit-or-miss, and when you miss, you can end up with some truly atrocious pieces. When I was a kid, Mega Bloks was the most common LEGO alternative (in the US, at least), and all of those pieces are terrible. The colors are washed out compared to LEGO, the plates aren't as rigid, and some of the bricks are a mess of scratches and dents.
Consistency is even a problem within individual sets. I also had a couple of LEGO-compatible sets that were made by Hasbro in the mid-2000s, and the part quality is excellent. The pieces have held up really well over the years, and Hasbro's yellow pieces are indistinguishable from LEGO's yellow. Hasbro came correct... except for their wheels, which are the sort of hideous dirt-cheap abomination that even Mega Bloks never stooped to.
Some brands are awful. Some brands are great. Some brands (like Hasbro) somehow managed to put "awful" and "great" inside the very same box. A brand's quality may change wildly over time. Some brands come out with their own extensions to the LEGO "standard," and then never provide a complete system of compatible parts. Sticking to name-brand LEGO offers a much more predictable experience.