Because of how it is priced relative to the other tiers. It is significantly closer in price to the highest tier ($7 gap) despite being almost identical to the lowest tier in terms of features ($11 gap). The only meaningful difference is ads. It’s very clear that Netflix is trying to price out customers who are unwilling or uninterested in paying for all the features limited to the highest tier (4K, double the simultaneous streams, better audio, etc) and force them on to the cheaper, ad-supported plan instead because they believe they can make more money this way.
Additionally, the middle tier is priced closer to the highest tier to imply that the highest tier is great value (because it has so many extra features for a smaller price increase than between the lowest tier and the middle tier). This is a classic retail strategy designed to trick consumers into spending more money than they originally needed or wanted to because “it’s better value”. Consumers often conflate “better value” with “saving money”, even though they are doing the complete opposite when they pay $7 more for features they didn’t initially care about at all.