Why do they invest effort in making this harder? They could just make it easy for at least a single model they keep producing and capture the whole niche of customers (and also get free advertisement by "the word of mouth" on HN and alike places) with no additional investment.

The most charitable interpretation: Rootability tends to be inverse to hackability. There's incentive for companies to increase security, to prevent their IoT stuff from getting turned into botnets. But there's nearly no incentive to preserve rootability. So it's just locked down to an increasing degree.

Less charitable: they don't want you to root it for various corporate reasons